Communication system, communication apparatus, and packet length control method

ABSTRACT

There is provided a communication apparatus includes a soft decision data detection section which detects soft decision data from a received digitally modulated signal; an indicator calculation section which calculates an indicator indicating a degree of variation in the soft decision data detected by the soft decision data detection section; and a packet length control section which controls a packet length in accordance with the indicator calculated by the indicator calculation section.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a communication system, a communicationapparatus, and a packet length control method.

2. Description of the Related Art

When data is transmitted/received between a transmitter and a receiver,the data may not be transmitted/received correctly due to communicationchannel characteristics, communication path congestion, and the like. Insuch a case, the transmitter may repeatedly perform retransmissionprocessing of the data until the data is correctly transmitted to thereceiver. However, if the size of the data is large, it is oftendifficult to transmit the data correctly even though the retransmissionprocessing is repeated many times. In such a case, the transmitterdivides the large size data into small size data, and then transmits thedata. The small size data is more likely to be transmitted correctlythan the large size data. Therefore, by dividing the data into smalldata and transmitting the small data, the data can be transmittedcorrectly.

When transmitting a plurality of divided data, the transmitter generatesa packet individually for each divided data. The transmitter performsmodulation on each packet by using a predetermined modulation method,and transmits the packets to the receiver. As the predeterminedmodulation method, for example, a phase shift keying method, anamplitude modulation method, and the like are used. The receiverreceives a modulated signal transmitted from the transmitter,demodulates the modulated signal by using a predetermined method (forexample, refer to Japanese Patent Application Laid-Open No.2008-294730), and extracts the divided data included in each packet.When having received all the divided data, the receiver combines thedivided data to restore the original data. By using a method asdescribed above, data can be transmitted from the transmitter to thereceiver with a relatively high possibility even if the communicationenvironment is bad or the like.

SUMMARY OF THE INVENTION

However, information necessary for communication, such as a header isadded to each packet. In addition, a predetermined processing time isgenerated before and after transmission of each packet. Therefore,transmitting a plurality of small size data by using a plurality ofpackets may require longer time to transmit data than transmitting largesize data by using one packet. However, considering that the large sizedata is retransmitted many times, in many cases, data can be transmittedin a shorter time by dividing data and transmitting the divided data. Ofcourse, if the size of the divided data is large, data may not betransmitted in a short time even when the data is divided. Therefore, amethod for dividing data into an appropriate size while considering thetime used to transmit one packet, dependency of transmission error rateon data size, and the like is desired.

In light of the foregoing, it is desirable to provide a novel andimproved communication system, communication apparatus, and packetlength control method which can determine appropriate divided data sizeconsidering transmission error rate with a relatively small processingload.

According to an embodiment of the present invention, there is provided acommunication apparatus which includes a soft decision data detectionsection which detects soft decision data from a received digitallymodulated signal; an indicator calculation section which calculates anindicator indicating a degree of variation in the soft decision datadetected by the soft decision data detection section; and a packetlength control section which controls a packet length in accordance withthe indicator calculated by the indicator calculation section.

Furthermore, the indicator calculation section may include a standarddeviation calculation section which calculates a standard deviation ofthe soft decision data of some or all of packets included in thedigitally modulated signal; and an absolute value average calculationsection which calculates an average value of absolute values of the softdecision data of some or all of the packets, and the indicatorcalculation section calculates the indicator based on the standarddeviation calculated by the standard deviation calculation section andthe average value calculated by the absolute value average calculationsection.

Furthermore, the indicator calculation section may calculate theindicator S=σ/(2*E) based on the standard deviation σ and the averagevalue E.

Furthermore, the packet length control section may shorten the packetlength when the indicator S is large, and lengthens the packet lengthwhen the indicator S is small.

Furthermore, the packet length control section may refer to apredetermined table configured to associate predetermined packet lengthswith the indicators S, select a predetermined packet length inaccordance with the indicator S calculated by the indicator calculationsection, and packetize data so that the data has the predeterminedpacket length.

Furthermore, the packet length control section may include a packetlength calculation section which uses a predetermined calculationformula for calculating an expected value of a time used to transmit alldata of n bits which is transmitted by dividing the data into packets ofk bits (n>k) when a communication condition corresponds to the indicatorS, and calculates a packet length k which makes the expected valueminimum; and a packetizing section which divides transmission data basedon the packet length k calculated by the packet length calculationsection in accordance with the indicator S calculated by the indicatorcalculation section and the number of bits n of the transmission data,and packetizes the divided transmission data.

Furthermore, the communication apparatus may further include anindicator transmission section which transmits information of theindicator calculated by the indicator calculation section to atransmission source of the digitally modulated signal.

Furthermore, the communication apparatus may further include acommunication control section which stops data transmission processingwhen the indicator calculated by the indicator calculation section isgreater than a predetermined threshold value, and re-starts the datatransmission processing when the indicator is smaller than apredetermined threshold value.

According to an embodiment of the present invention, there is provided acommunication apparatus which includes an indicator reception sectionwhich receives an indicator when soft decision data of a digitallymodulated signal is detected in a transmission destination of thedigitally modulated signal transmitted by the communication apparatus,and the indicator representing a degree of variation in the softdecision data is calculated and transmitted to the communicationapparatus; and a packet length control section which controls a packetlength in accordance with the indicator received by the indicatorreception section.

Furthermore, the packet length control section may shorten the packetlength when the indicator S is large, and lengthens the packet lengthwhen the indicator S is small.

Furthermore, the packet length control section may refer to apredetermined table configured to associate predetermined packet lengthswith the indicators, select a predetermined packet length in accordancewith the indicator calculated by the indicator calculation section, andpacketize data by the predetermined packet length.

Furthermore, the packet length control section may include a packetlength calculation section which uses a predetermined calculationformula for estimating an expected value of a time used to transmit alldata of n bits which is transmitted by dividing the data into packets ofk bits (n>k) when a communication condition corresponds to the indicatorS, and calculates a packet length k which makes the expected valueminimum; and a packetizing section which divides transmission data basedon the packet length k calculated by the packet length calculationsection in accordance with the indicator S calculated by the indicatorcalculation section and the number of bits n of the transmission data,and packetizes the divided transmission data.

Furthermore, the communication apparatus may further include acommunication control section which transmits a digitally modulatedsignal including a predetermined packet when the indicator received bythe indicator reception section is greater than a predeterminedthreshold value.

According to an embodiment of the present invention, there is provided acommunication system which includes a first communication deviceincluding a signal transmission section which transmits a digitallymodulated signal to a second communication device, an indicatorreception section which receives an indicator transmitted from thesecond communication device, and a packet length control section whichcontrols a packet length of a packet to be transmitted to the secondcommunication device in accordance with the indicator received by theindicator reception section; and the second communication deviceincluding a signal reception section which receives the digitallymodulated signal transmitted from the first communication device, a softdecision data detection section which detects soft decision data fromthe digitally modulated signal received by the signal reception section,an indicator calculation section which calculates an indicatorindicating a degree of variation in the soft decision data detected bythe soft decision data detection section, and an indicator transmissionsection which transmits the indicator calculated by the indicatorcalculation section to the first communication device.

According to another embodiment of the present invention, there isprovided a packet length control method, including the steps ofdetecting soft decision data from a received digitally modulated signal;calculating an indicator indicating a degree of variation in the softdecision data detected in the step of detecting soft decision data; andcontrolling a packet length in accordance with the indicator calculatedin the step of calculating an indicator.

According to another embodiment of the present invention, there isprovided a packet length control method, including the steps ofreceiving an indicator when soft decision data of a transmitteddigitally modulated signal is detected in a transmission destination ofthe digitally modulated signal, and the indicator representing a degreeof variation in the soft decision data is calculated and transmitted toa transmission source of the digitally modulated signal; and controllinga packet length in accordance with the indicator received in the step ofreceiving an indicator.

According to another embodiment of the present invention, there isprovided a packet length control method, including the steps of by afirst communication device, transmitting a digitally modulated signal toa second communication device; by the second communication device,receiving the digitally modulated signal transmitted from the firstcommunication device; detecting soft decision data from the digitallymodulated signal received in the step of receiving the digitallymodulated signal; calculating an indicator indicating a degree ofvariation in the soft decision data detected in the step of detectingsoft decision data; transmitting the indicator calculated in the step ofcalculating an indicator to the first communication device; by the firstcommunication device, receiving the indicator transmitted from thesecond communication device; and controlling a packet length of a packetto be transmitted to the second communication device in accordance withthe indicator received in the step of receiving the indicator.

According to the embodiments of the present invention described above,it is possible to determine appropriate divided data size consideringtransmission error rate with a relatively small processing load.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a configuration example of a communication system accordingto a first embodiment of the present invention;

FIG. 2 shows an outline of a retransmission control method and adata-divided transmission method;

FIG. 3 shows an example of a signal waveform when transmitting adigitally modulated signal;

FIG. 4 shows an example of a signal waveform when receiving thedigitally modulated signal;

FIG. 5 shows a distribution of soft decision data (amplitude values)obtained from a received signal;

FIG. 6 shows a histogram of soft decision data (amplitude values)obtained from a received signal;

FIG. 7 shows a packet length modification method according to the firstembodiment;

FIG. 8 shows a functional configuration example of a communicationapparatus according to the first embodiment;

FIG. 9 shows a relationship between indicators calculated by thecommunication apparatus according to the first embodiment and bit errorrate;

FIG. 10 shows a relationship between indicators calculated by thecommunication apparatus according to the first embodiment and bit errorrate, and a configuration example of an indicator table;

FIG. 11 shows a detailed functional configuration of a packet lengthmodification section according to a modified example of the firstembodiment;

FIG. 12 shows a packet length determination method according to themodified example of the first embodiment;

FIG. 13 shows an example of an indicator table determined based on thepacket length determination method according to the modified example ofthe first embodiment;

FIG. 14 shows a relationship between packet lengths determined based onthe packet length determination method according to the modified exampleof the first embodiment and indicators;

FIG. 15 shows a packet length modification method according to a secondembodiment of the present invention;

FIG. 16 shows a functional configuration example of a communicationapparatus according to the second embodiment;

FIG. 17 shows a functional configuration example of a packet lengthmodification section according to the second embodiment;

FIG. 18 shows a configuration example of a transmission packet accordingto the second embodiment;

FIG. 19 shows a functional configuration example of a communicationapparatus according to a third embodiment of the present invention;

FIG. 20 shows a hardware configuration example of the communicationapparatus according to the third embodiment;

FIG. 21 shows a register configuration of the communication apparatusaccording to the third embodiment;

FIG. 22 shows a circuit configuration example of a statisticalprocessing section according to the third embodiment;

FIG. 23 shows a configuration example of an arithmetic processingsection according to the third embodiment;

FIG. 24 shows a hardware configuration example of a communicationapparatus according to a modified example of the third embodiment;

FIG. 25 shows a process flow of the communication apparatus according tothe modified example of the third embodiment;

FIG. 26 shows a process flow of the communication apparatus according tothe modified example of the third embodiment;

FIG. 27 shows a circuit configuration example of a soft decision datageneration section included in the communication apparatuses accordingto each embodiment of the present invention;

FIG. 28 shows a circuit configuration example of a soft decision datageneration section included in the communication apparatuses accordingto each embodiment of the present invention; and

FIG. 29 shows a circuit configuration example of a soft decision datageneration section included in the communication apparatuses accordingto each embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will bedescribed in detail with reference to the appended drawings. Note that,in this specification and the appended drawings, structural elementsthat have substantially the same function and structure are denoted withthe same reference numerals, and repeated explanation of thesestructural elements is omitted.

[Flow of Description]

Here, a flow of the description below related to embodiments of thepresent invention will be briefly described.

First, a configuration of a communication system 10 according to a firstembodiment of the present invention will be briefly described withreference to FIG. 1. Next, a retransmission control method and adata-divided transmission method will be described with reference toFIG. 2. Next, an estimation method of transmission error rate accordingto the first embodiment will be described with reference to FIGS. 3 to6. Next, a packet length modification method according to the firstembodiment will be described with reference to FIG. 7.

Next, a functional configuration of each communication apparatusaccording to the first embodiment will be described with reference toFIG. 8. In the above description, a determination method of an indicatorcorresponding to the transmission error rate will be described withreference to FIGS. 9 and 10. Next, with reference to FIGS. 11 to 14, acalculation method of the indicator corresponding to the transmissionerror rate will be described, and a method for appropriately modifying apacket length based on the indicator calculated by using the calculationmethod.

Next, a packet length modification method according to a secondembodiment of the present invention will be described with reference toFIG. 15. Next, a functional configuration of each communicationapparatus according to the second embodiment will be described withreference to FIGS. 16 and 17. Next, a configuration of a transmissionpacket and an indicator transmission method according to the secondembodiment will be described with reference to FIG. 18.

Next, a functional configuration of each communication apparatusaccording to a third embodiment of the present invention will bedescribed with reference to FIG. 19. Next, a hardware configuration ofeach communication apparatus according to the third embodiment will bedescribed with reference to FIGS. 20 to 23. Next, a functionalconfiguration of each communication apparatus according to a modifiedexample of the third embodiment will be described with reference to FIG.24. Next, a flow of data read/write processing according to the thirdembodiment will be described with reference to FIG. 25. Next, a packettransmission method according to the third embodiment will be describedwith reference to FIG. 26. And then, a detection method of soft decisiondata will be described with reference to FIGS. 27 to 29.

(Description Items)

1: First Embodiment

-   -   1-1: System configuration    -   1-2: Retransmission control method and data-divided transmission        method    -   1-3: Estimation method of transmission error rate    -   1-4: Packet length modification method    -   1-5: Functional configuration of communication apparatus        -   1-5-1: Functional configuration of first communication            apparatus 100        -   1-5-2: Functional configuration of second communication            apparatus 200        -   1-5-3: (Modified example) How to obtain packet length

2: Second Embodiment

-   -   2-1: Packet length modification method    -   2-2: Functional configuration of communication apparatus        -   2-2-1: Functional configuration of first communication            apparatus 300        -   2-2-2: Functional configuration of second communication            apparatus 400

3: Third Embodiment

-   -   3-1: Functional configuration of communication apparatus        -   3-1-1: Functional configuration of first communication            apparatus 500        -   3-1-2: Functional configuration of second communication            apparatus 600    -   3-2: Hardware configuration of each communication apparatus    -   3-3: (Modified example) Hardware configuration of each        communication apparatus    -   3-4: Flow of data read/write processing    -   3-5: Transmission method including dynamic packet length        modification processing

4: Addendum

-   -   4-1: Detection method of soft decision data        -   4-1-1: Circuit configuration of sampling clock generation            section 800        -   4-1-2: Circuit configuration of soft decision baseband            signal generation section 820        -   4-1-3: Circuit configuration of data generation section 830

1: First Embodiment

The first embodiment of the present invention will be described. Thisembodiment relates to a method in which an indicator to evaluatecommunication condition is calculated and packet length is dynamicallymodified based on the indicator.

When performing wireless communication, data may be divided into aplurality of blocks and transmitted. This is because, the larger theamount of data, the higher the possibility that an error occurs in adata stream is. When an error occurs in a data series, the data isrepeatedly retransmitted until correct data is received, and finally,the correct data is received. However, when a communication pathcondition is bad, correct data is difficult to be received even if thedata is retransmitted many times. As a result, the communication timebecomes long, so that users' convenience is hampered. Therefore, amethod is used in which data is divided into small blocks andtransmitted so that correct data can be received even when acommunication path condition is bad.

However, when the data is divided into blocks, a time interval isgenerated after a block is transmitted and before the next block istransmitted. Therefore, when the data is divided into too many blocks,the time intervals increase. As a result, the communication time becomeslong as the time intervals increase, so that users' convenience ishampered. Therefore, to increase users' convenience by shortening thecommunication time, it is desired to appropriately evaluate thecommunication path condition, divide the data into appropriate lengthsin accordance with the communication path condition, and transmit thedata. Responding to a request as described above, this embodimentprovides a method for appropriately evaluating a communication pathcondition, and a method for dividing data into appropriate lengths inaccordance with the evaluation result.

1-1: System Configuration

First, a configuration of a communication system 10 according to thisembodiment will be briefly described with reference to FIG. 1. FIG. 1 isan illustration showing a schematic configuration of the communicationsystem 10 according to this embodiment. Basic configuration ofcommunication systems 20 and 30 described below is substantially thesame as that of the communication system 10.

As shown in FIG. 1, the communication system 10 includes a firstcommunication apparatus 100 and a second communication apparatus 200.The first communication apparatus 100 can transmit data to the secondcommunication apparatus 200. Further, the second communication apparatus200 can transmit data to the first communication apparatus 100. Acommunication path through which data is transmitted from the firstcommunication apparatus 100 to the second communication apparatus 200 isrepresented as CH12. In the same way, a communication path through whichdata is transmitted from the second communication apparatus 200 to thefirst communication apparatus 100 is represented as CH21.

Usually, channel characteristics of the communication channel CH12 andchannel characteristics of the communication channel CH21 are different.However, when the first communication apparatus 100 and the secondcommunication apparatus 200 are installed in places with a fine view,the channel characteristics of the communication channels CH12 and CH21may be assumed to be the same. In the first embodiment, an evaluationmethod of the communication condition and a data division method basedon an evaluation result of the evaluation method will be describedpresuming that the channel characteristics of the communication channelsCH12 and CH21 can be assumed to be the same. Of course, these methodscan be extended to deal with a case in which the channel characteristicsof the communication channels CH12 and CH21 are different. Suchextension will be explained in a second and third embodiments describedbelow.

1-2: Retransmission Control Method and Data-Divided Transmission Method

Here, a data retransmission control method and a data-dividedtransmission method will be briefly described with reference to FIG. 2.FIG. 2 is an illustration for explaining the data retransmission controlmethod and the data-divided transmission method. Packet configurationsand communication times shown in FIG. 2 are an example.

Part (A) in FIG. 2 shows a packet configuration for transmitting theoriginal data to be divided without dividing the original data. Also,part (A) shows a communication time t0 used to transmit the data withoutdividing the data. Part (B) in FIG. 2 shows a communication time t1 usedto retransmit the data without dividing the data. On the other hand,part (C) in FIG. 2 shows a packet configuration for transmitting thedata by dividing the data into halves and a communication time t2 usedto retransmit one of divided blocks. In a similar way, part (D) in FIG.2 shows a packet configuration for transmitting the data by dividing thedata into quarters and a communication time t3 used to retransmit one ofdivided blocks.

As shown in part (A), a case in which data of N bytes is transmitted isconsidered. As described above, to transmit this data, the communicationtime t0 is used, which is a total time of data transmission time, headertransmission time, and time interval. If an error is detected in a topportion of the data which is located just after the header, as acountermeasure, it is considered that retransmission (B), retransmissionby dividing data into halves (C), or retransmission by dividing datainto quarters (D) is performed. As shown in part (B), when performingretransmission, the time t1 which is used to transmit data successfullyis two times (t1=2*t0) the time used when not performing retransmission.

On the other hand, when transmitting data by dividing the data intohalves (C), only the first half of the divided data has to betransmitted, so that the time t2 used to transmit the data is only 1.5times the time used when not performing retransmission. However, thetime used to transmit the headers and the time intervals are three timeslonger. However, the total time t2 which is the sum of the time used totransmit the headers and the time intervals is shorter than the timeused when retransmitting the data without dividing the data (A) by atime Δt12.

When transmitting data by dividing the data into quarters (D), only thefirst half of the divided data has to be transmitted, so that the timet3 used to transmit the data is only 1.25 times the time used when notperforming retransmission. However, the time used to transmit theheaders and the time intervals are four times longer. Therefore, thetotal time t3 which is the sum of the time used to transmit the headersand the time intervals is longer than the time used when retransmittingthe data without dividing the data (A). From these results, in theexample of FIG. 2, it is found that, when transmitting data by dividingthe data into halves, the total time used for transmission is theshortest.

In this way, an appropriate data length is determined depending on thelength of the data to be transmitted, the lengths of the header and thetime interval, the data division number, the number of retransmissiontimes, and the like. Therefore, instead of simply repeatingretransmission in accordance with a communication path condition, bytransmitting the data by dividing the data into appropriate lengths inaccordance with the communication path condition, the totalcommunication time can be shortened. Therefore, hereinafter, a methodfor appropriately evaluating the communication path condition, and amethod for determining the length of the divided data by using theevaluation result will be described in order. The communication pathcondition can be evaluated by estimating transmission error rate.Therefore, a method in which the transmission error rate can beestimated with relatively simple configuration will be described.

1-3: Estimation Method of Transmission Error Rate

First, refer to FIG. 3. FIG. 3 schematically shows a signal waveform ofa digitally modulated signal when being transmitted. The digitallymodulated signal is not affected by the communication channel CH12 whenbeing transmitted. Therefore, an amplitude value of the digitallymodulated signal is an amplitude A or an amplitude B. If thecommunication channel CH12 is an ideal transmission path without anynoise, a signal waveform of the digitally modulated signal when beingreceived is as shown in FIG. 3. However, actually, noises are added tothe digitally modulated signal in the communication channel CH12, sothat the signal waveform of the digitally modulated signal when beingreceived is as shown in FIG. 4.

FIG. 4 schematically shows a signal waveform of the digitally modulatedsignal when being received. The signal waveform shown in FIG. 4 can beobtained by performing analog-digital conversion on the received signal,or performing predetermined soft decision processing on the receivedsignal. Specifically, FIG. 4 is an example of a signal waveformrepresented by soft decision data (hereinafter also referred to as“amplitude value”) of the digitally modulated signal when beingreceived. The horizontal axis is a time axis representing the number ofdata clock cycles. The signal waveform of FIG. 4 is different from thatof FIG. 3, and includes a number of values between the amplitude A andthe amplitude B by being affected by noises in the transmission path.

The value of the soft decision data represents the probability of harddecision data obtained by determining the sign of each amplitude value.Therefore, it is difficult to restore the original data correctly from asignal waveform including large changes in amplitude value over time asshown in FIG. 4. For example, when the soft decision data is near zero,it is difficult to determine whether the hard decision data, which isobtained from the sign of the soft decision data, is correct or not.However, by statistically analyzing the soft decision data, it ispossible to estimate probability of transmission error in thecommunication path CH12. For example, when enlarging the time range ofthe signal waveform shown in FIG. 4, a distribution diagram as shown inFIG. 5 can be obtained.

As will be appreciated from the distribution diagram in FIG. 5, thereare two centers of distribution (center of distribution A, center ofdistribution B) in the soft decision data obtained from the receiveddigitally modulated signal. There are two distributions (distribution A,distribution B) from each center of distribution. However, a set of thesoft decision data included in the distribution A and a set of the softdecision data included in the distribution B are not independent sets.Therefore, when representing more correctly, there is one distributionhaving two centers of distribution A and B. To clarify thecharacteristics of the distribution, FIG. 6 shows a histogram of thesoft decision data. In FIG. 6, the vertical axis is detection frequencyof each soft decision data, and the horizontal axis is the value of thesoft decision data.

As shown in FIG. 6, from the distribution diagram shown in FIG. 5, ahistogram having peaks at the amplitude value corresponding to thecenter of distribution A and the amplitude value corresponding to thecenter of distribution B can be obtained. A chain line and a shaded areain FIG. 6 are added for explanation. As described above, the harddecision data is obtained by determining the sign of the soft decisiondata. Therefore, when soft decision data which should have a positivesign has a negative sign, hard decision data obtained based on the softdecision data will be an error. In the histogram of FIG. 6, when a pointof soft decision data which should be included in the distribution Adistributed from the center of distribution A is included in thedistribution B distributed from the center of distribution B, the pointof the soft decision data will be an error.

Therefore, if a ratio in which a set of soft decision data correspondingto the above error occupies in the entire set of soft decision datadistributed from each center of distribution can be calculated, thetransmission error rate can be calculated. An entire amount of the softdecision data can be calculated by the area of the histogram. On theother hand, to estimate an amount of soft decision data corresponding tothe error, some technique is used. Here, refer to the histogram of FIG.6 again. The histogram of FIG. 6 has an approximate shape formed bycombining a normal distribution (hereinafter “normal distribution A”)centered on the center of distribution A and a normal distribution(hereinafter “normal distribution B”) centered on the center ofdistribution B. The histogram of FIG. 6 has a mirror symmetric shapewith respect to the amplitude value 0.

Therefore, by approximating the histogram by the normal distribution Acentered on the center of distribution A, and calculating the area inthe approximated normal distribution A protruding to an area having anamplitude value smaller than the amplitude value 0 (an area near thecenter of distribution B), an amount of soft decision data correspondingto the error can be estimated from the protruding area. A shaded areashown in FIG. 6 is the protruding area of the normal distribution A (andnormal distribution B) fitted to the histogram. Therefore, by obtaininga ratio between the area surrounded by the histogram and the shadedarea, the transmission error rate can be estimated. Conversely, anindicator corresponding to the transmission error rate can be calculatedbased on variance value of the soft decision data obtained from a partof a histogram approximated by a normal distribution based on one ofcenters of distribution and a position of the center of distribution(average value of each distribution).

Usually, to obtain the transmission error rate of the received signal,the original data has to be known in advance by the receiver. Also, toobtain the transmission error rate with a high degree of accuracy, someamount of data may be required. However, the above average value andvariance value can be calculated without knowing the original data.Further, these values can be easily obtained by using data in a shortinterval. In view of these advantages, in this embodiment, a statisticalindicator is used, which is determined based on the above average valueand variance value and used to evaluate the communication quality of thetransmission path. Hereinafter, an entire flow from the receivingprocess of the signal used for calculating the indicator to thetransmitting process of the signal including the divided data will bedescribed, and apparatus configurations for realizing processing of eachprocess will be described in order.

1-4: Packet Length Modification Method

First, an entire process flow including packet length modificationprocessing according to this embodiment will be described with referenceto FIG. 7. FIG. 7 is an illustration showing the entire process flowincluding the packet length modification processing according to thisembodiment. The processing of each step shown in FIG. 7 is performed bythe first communication apparatus 100 or the second communicationapparatus 200 included in the communication system 10.

As shown in FIG. 7, first, data is transmitted from the firstcommunication apparatus 100 to the second communication apparatus 200(S102). The data transmitted in step S102 is arbitrary data. The data tobe transmitted is modulated by a predetermined modulation method, andtransmitted to the second communication apparatus 200 as a modulatedsignal. Next, the second communication apparatus 200 receives the data(modulated signal) transmitted from the first communication apparatus100 (S104). Next, the second communication apparatus 200 generates asoft decision data string from the received modulated signal (S106). Thesoft decision data string can be detected, for example, by using ananalog-digital converter or a soft decision data detection methoddescribed below.

Next, the second communication apparatus 200 performs predeterminedstatistical processing using the soft decision data string generated instep S106, and calculates an indicator for evaluating the communicationcondition in the communication channel CH12 (S108). This indicatorindicates a degree of variation in the soft decision data stringgenerated in step S106. As described above, an evaluation indicatorcorresponding to the transmission error rate can be calculated based onthe degree of variation (for example, variance value) in the softdecision data. In step S108, such an evaluation indicator is calculated.For example, an indicator which becomes a large value when thecommunication condition of the communication channel CH12 is bad, andbecomes a small value when the communication condition is good iscalculated.

Next, the second communication apparatus 200 modifies the packet lengthof the data series to be transmitted based on the indicator calculatedin step S108 (S110). For example, the second communication apparatus 200performs a modification to shorten the packet length when the value ofthe indicator is large, and performs a modification to lengthen thepacket length when the value of the indicator is small. The secondcommunication apparatus 200 divides the data by the modified packetlength, and transmits the data to the first communication apparatus 100by packetizing each divided data (S112). The packet of each divided data(hereinafter, divided packet) is modulated by a predetermined modulationmethod, and transmitted as a modulated signal. The data of the dividedpacket is received by the first communication apparatus 100 (S114).

When receiving all the divide packets, the first communication apparatus100 combines the divided packets to restore the original data. By usingsuch a method, the packet length in the transmission system can bedynamically modified in accordance with the transmission path condition.As a result, even when channel characteristics of the communicationchannel CH12 change, data can be dividedly transmitted by an appropriatepacket length, so that the communication time is shortened. As a result,the convenience of the user can be improved.

Regarding the packet length modification method according to thisembodiment, the general process flow has been described. The methoddescribed above can be applied when the channel characteristics of thecommunication channels CH12 and CH21 can be assumed to be the same. Acase in which the channel characteristics of the communication channelsCH12 and CH21 are different will be described in detail in a secondembodiment and a third embodiment below. Next, functional configurationsof each communication apparatus for realizing the above method will bedescribed.

1-5: Functional Configuration of Communication Apparatus

Hereinafter, functional configurations of the first communicationapparatus 100 and the second communication apparatus 200 included in thecommunication system 10 which can realize the above method will bedescribed in detail. The technical features of this embodiment aremainly included in the functional configuration of the secondcommunication apparatus 200.

(1-5-1: Functional Configuration of First Communication Apparatus 100)

First, a functional configuration of the first communication apparatus100 will be described with reference to FIG. 8. FIG. 8 shows afunctional configuration example of the first communication apparatus100 according to this embodiment.

As shown in FIG. 8, the first communication apparatus 100 mainlyincludes a modulation section 102, a signal transmission section 104, asignal reception section 106, and a demodulation section 108. Asdescribed above, a main role of the first communication apparatus 100according to this embodiment is to transmit data to the secondcommunication apparatus 200, and receive data transmitted from thesecond communication apparatus 200.

First, transmission data is input into the modulation section 102. Inthe modulation section 102, the transmission data is modulated based ona predetermined modulation method, and a modulated signal is generated.As the predetermined modulation method, for example, the phase shiftkeying method, the amplitude modulation method, and the like are used.The modulated signal generated by the modulation section 102 is inputinto the signal transmission section 104. When the modulated signal isinput, the signal transmission section 104 transmits the modulatedsignal to the second communication apparatus 200.

As described above, in the second communication apparatus 200, theindicator is calculated based on the received modulated signal, and thedivided packet is generated by a packet length modified based on theindicator. This divided packet is transmitted from the secondcommunication apparatus 200 to the first communication apparatus 100 asa modulated signal. The first communication apparatus 100 receives themodulated signal including the divided packet by the signal receptionsection 106. The modulated signal received by the signal receptionsection 106 is input into the demodulation section 108. In thedemodulation section 108, divided data included in each divided packetis read, and all the divided data are combined and the original data isrestored.

So far, the functional configuration of the first communicationapparatus 100 has been described.

(1-5-2: Functional Configuration of Second Communication Apparatus 200)

Next, a functional configuration of the second communication apparatus200 will be described with reference to FIG. 8. FIG. 8 shows afunctional configuration example of the second communication apparatus200 according to this embodiment.

As shown in FIG. 8, the second communication apparatus 200 mainlyincludes a signal reception section 202, a soft decision data generationsection 204, an indicator calculation section 206, a communicationcontrol section 208, a packet length modification section 210, amodulation section 212, and a signal transmission section 214. Thepacket length modification section 210 includes a packet lengthselection section 232 and a packetizing section 234.

As described above, when the modulated signal is transmitted from thefirst communication apparatus 100, the modulated signal is received bythe signal reception section 202. The modulated signal received by thesignal reception section 202 is input into the soft decision datageneration section 204. When the modulated signal is input, the softdecision data generation section 204 generates soft decision data fromthe modulated signal. The soft decision data generation section 204detects each bit value as a value having two or more bits (soft decisiondata value) by using, for example, analog-digital conversion or a softdecision data detection method (refer to FIGS. 27, 28, and 29) describedbelow. For example, the soft decision data generation section 204 candetect a soft decision data string having amplitude values as shown inFIG. 4.

The soft decision data string generated by the soft decision datageneration section 204 is input into the indicator calculation section206. When the soft decision data string is input, the indicatorcalculation section 206 counts the number of data values (hereinafter,parameter ERNUM) included in the soft decision data string. Theindicator calculation section 206 also calculates absolute values ofeach soft decision data. The indicator calculation section 206 sums upthe absolute values of each soft decision data and calculates anabsolute value sum (ERABS). Further, the indicator calculation section206 calculates an average of the absolute values (hereinafter, absolutevalue average AVE) by using the parameter ERNUM and the absolute valuesum ERABS. Also, the indicator calculation section 206 calculates squarevalues of each soft decision data and calculates the sum of the squarevalues (hereinafter, square value sum ERSQA).

The indicator calculation section 206 calculates a standard deviation(STDEV) of the soft decision data string by using the parameter ERNUM,the square value sum ERSQA, and the absolute value sum ERABS. Theabsolute value average AVE and the standard deviation STDEV arecalculated based on the formulas (1) and (2) below. When the absolutevalue average AVE and the standard deviation STDEV are calculated, theindicator calculation section 206 calculates the indicator (SDI) basedon the formula (3) below.

AVE=ERABS/ERNUM  (1)

STDEV={ERSQA/ERNUM−(ERABS/ERNUM)²}^(0.5)  (2)

SDI=STDEV/(2*AVE)  (3)

As already described with reference to FIG. 6, there is a correlationbetween the distribution of the soft decision data string and thetransmission error rate. Therefore, the indicator SDI based on thestandard deviation STDEV (or the variance value STDEV2) corresponding tothe degree of variation in the soft decision data string can be used asan evaluation value for evaluating a communication condition of atransmission path. Actually, there is a relationship as shown in FIG. 9between the indicator SDI and a bit error rate (bER). From FIG. 9, itmay be understood that there is a one-on-one relationship between theindicator SDI and the bit error rate (bER).

Here, the relationship between the indicator SDI and the bit error rate(bER) will be examined in more detail. As already described above, thebit error rate bER can be estimated from the area of the histogram shownin FIG. 6. Specifically, the value of the bit error rate bER can beestimated by obtaining an area ratio of a portion, which protrudes tothe negative (or positive) area, in the normal distribution (averagevalue E, variance σ²) of the soft decision data whose center ofdistribution is located in the positive (or negative) area. Consideringthe above, when representing the value of the bit error rate bER byusing the above indicator SDI, the value is represented by the formula(4) below. The term erfc { . . . } represents a cumulative errorcomplementary function.

$\begin{matrix}\begin{matrix}{{bER} = {\left( {1/2} \right)*{erfc}\left\{ {1/\left( {2^{1.5}*{SDI}} \right)} \right\}}} \\{= {\left( {1/2} \right)*{erfc}\left\{ {E/\left( {2^{0.5}*\sigma} \right)} \right\}}}\end{matrix} & (4)\end{matrix}$

Also from the above formula (4), it is possible to understand that thereis a one-on-one relationship between the indicator SDI and the bit errorrate (bER). The indicator SDI calculated in that way by the indicatorcalculation section 206 is input into the communication control section208 and the packet length modification section 210 (refer to FIG. 8).

When the indicator SDI is input, the communication control section 208stops data transmission or re-starts the stopped data transmission inaccordance with the value of the indicator SDI. For example, when theindicator SDI is greater than a predetermined value, the communicationcontrol section 208 determines that the communication environmentdeteriorates, and inputs a control signal for stopping the datatransmission into the signal transmission section 214. On the otherhand, when the indicator SDI is smaller than a predetermined value, thecommunication control section 208 determines that the communicationenvironment improves, and inputs a control signal for re-starting thedata transmission into the signal transmission section 214.

The above predetermined values are values of the indicator SDI forcoping with a communication condition in which, even when modifying thepacket length, data is difficult to transmit in a predetermined timeperiod. By making such a configuration, it is possible to stop the datatransmission when the communication environment deteriorates, andre-start the data transmission after a random or a predetermined timehas passed, or after the communication environment has improved. Whenthere are many communication devices nearby which use the same channel,data transmission timing may need to be appropriately adjusted. In sucha situation, when each communication device calculates the aboveindicator SDI, and controls transmission timing based on the indicatorSDI, the data transmission timing can be adjusted more efficiently.

When the indicator SDI is input, the packet length modification section210 modifies the packet length in accordance with the value of theindicator SDI, and generates a divided packet by the modified packetlength. As described above, the packet length modification section 210includes the packet length selection section 232 and the packetizingsection 234. The packet length selection section 232 is a means forselecting an appropriate packet length in accordance with the inputindicator SDI. On the other hand, the packetizing section 234 is a meansfor dividing transmission data by the packet length selected by thepacket length selection section 232, and generating a divided packet bypacketizing the divided transmission data.

First, the indicator SDI is input into the packet length selectionsection 232. When the indicator SDI is input, the packet lengthselection section 232 refers to the indicator SDI and an indicator table(refer to FIG. 10) in which the indicators SDI and predetermined packetlengths are associated with each other, and selects an appropriatepacket length. Here, a configuration of the indicator table will bedescribed with reference to FIG. 10. FIG. 10 is an illustration showinga configuration example of the indicator table according to thisembodiment.

The above indicator table is set based on the relationship between theindicator SDI and the bit error rate (bER). As already described above,there is a one-on-one relationship between the indicator SDI and the biterror rate bER. When the bit error rate bER is determined, an erroroccurrence rate P (without error correction) when a packet of apredetermined size is transmitted can be estimated based on the formula(5) below. In the formula (5) below, p is the bit error rate bERcalculated by the above formula (4). On the other hand, the erroroccurrence rate P is a sum of probabilities in which k (k>0) data piecesbecome error in n data pieces in the packet. The values 1024 [Byte] to64 [Byte] represented in the upper most fields indicate packet lengthsof the transmission data.

$\begin{matrix}{{P \cong {\sum\limits_{k = 1}^{n}{{\,_{n}C}{{{}_{}^{}{}_{}^{}}\left( {1 - p} \right)}^{n - k}}}} = {1 - \left( {1 - p} \right)^{n}}} & (5)\end{matrix}$

Although the indicator table of table (B) is set from the erroroccurrence rate P of table (A), an allowable error occurrence rate P isdetermined depending on an application, a communication method, and thelike. Therefore, the indicator table is set to appropriately adjustedvalues depending on the application, the communication method, and thelike so that the packet length is short when the value of the erroroccurrence rate P is large. For example, the indicator table is set byusing values shown in table (B). By using the indicator table set inthis way, it is expected that the total communication time used totransmit a large data can be shortened.

Refer to FIG. 8 again. The packet length selection section 232 refers tothe indicator table as described above, and selects an packet lengthcorresponding to the indicator SDI calculated by the indicatorcalculation section 206. The packet length information selected by thepacket length selection section 232 is input into the packetizingsection 234. Further, transmission data to be transmitted from thesecond communication apparatus 200 to the first communication apparatus100 is input into the packetizing section 234. When the packet lengthinformation and the transmission data are input, the packetizing section234 divides the transmission data based on the packet lengthinformation, and generates divided packet by packetizing eachtransmission data (divided data) having been divided. The divided packetgenerated by the packetizing section 234 is input into the modulationsection 212.

When the divided packet is input from the packet length modificationsection 210 (packetizing section 234), the modulation section 212modulates the divided packet based on a predetermined modulation method,and generates a modulated signal. The modulated signal generated by themodulation section 212 is input into the signal transmission section214. When the modulated signal is input, the signal transmission section214 transmits the input modulated signal to the first communicationapparatus 100. The signal transmission section 214 stops thetransmission of the modulated signal or re-starts the transmission inaccordance with the control signal input from the communication controlsection 208. The signal transmission section 214 may be configured totransmit a test packet for checking the communication condition in thetransmission path to the first communication apparatus 100 when thecontrol signal to stop the transmission is input from the communicationcontrol section 208.

So far, the functional configuration of the second communicationapparatus 200 has been described. As described above, the technicalfeature of this embodiment is a configuration in which the indicator SDIis calculated based on statistic values of the soft decision data stringobtained from the received modulated signal, and the packet length ismodified in accordance with the indicator SDI. By making such aconfiguration, it is possible to evaluate the communication condition inthe transmission path and generate the divided packet having anappropriate length according to the evaluation result, so that the totalcommunication time to be used for transmitting entire data can beshortened. As a result, the convenience of the user improves.

1-5-3: (Modified Example) How to Obtain Packet Length

So far, this embodiment has been described based on the method in whicha packet length corresponding to the indicator SDI is selected based ona predetermined indicator table. However, when the indicator SDI isobtained, an expected value of the total communication time used totransmit data of n bits by divided packets of k bits can be calculated.Accordingly, a method for determining an appropriate packet length fromthe expected value of the total communication time can be considered.Therefore, this method is proposed as a modified example of thisembodiment. When applying this method, the functional configuration ofthe packet length modification section 210 is modified as shown in FIG.11. FIG. 11 is an illustration showing a functional configurationexample of the packet length modification section 210 according to thismodified example.

As shown in FIG. 11, the packet length modification section 210 includesa packet length calculation section 236 and the packetizing section 234.In other words, the packet length selection section 232 is replaced bythe packet length calculation section 236. The functional configurationof the packetizing section 234 is the same as that of the firstembodiment. Therefore, hereinafter a method for calculating the packetlength by the packet length calculation section 236 will be described indetail.

To determine a packet length to reduce the total communication time tominimum, the header, the time interval, and the like have to beconsidered. Regarding the time lengths of the header, the time interval,and the like, there are a time t0 which is proportional to the packetlength and a time t1 which is constant and independent from the packetlength. The time t0, which is proportional to the packet length,includes the transmission time, an access time to an internal memory,and the like. For example, the total time to transmit a packet of 1 bitlength is defined as t0. On the other hand, the constant time t1, whichis used for every packet independent from the packet length, includes,for example, the time length of the header or the like.

When considering the time t0 and the time t1, the expected value of thetotal time used to transmit data of n bits by dividing the data intopackets of k bits can be calculated based on the formula (6) below. Notethat “p” represents the bit error rate bER, and [ . . . ] represents aninteger portion of . . . .

$\begin{matrix}{{\langle{T(k)}\rangle} = {{\left( {{kt}_{0} + t_{1}} \right){\left( {1 - p} \right)^{- k}\left\lbrack \frac{n}{k} \right\rbrack}} \cong {\left( \frac{t_{0} + {t_{1}/k}}{\left( {1 - p} \right)^{k}} \right)n}}} & (6)\end{matrix}$

When assuming that the time t0 is 400 [ns] which is proportional to thepacket length, the time t1 is 0.4 [ms] which is used for every packet,and the packet length n is 1024 [Byte], the expected values of the totalcommunication time calculated by using the above formula (6) are shownin FIG. 12. In FIG. 12, the vertical axis represents the time used totransmit a packet of length n, and the horizontal axis represents thepacket length. In FIG. 12, five graphs (A) to (E) which are obtained bychanging the value of indicator SDI are shown. In FIG. 12, the smallerthe value of the vertical axis is, the better. Therefore, the optimalpacket length is a packet length corresponding to the minimum point ofeach graph.

From the example of FIG. 12, it is understood that positions of theminimum points move to the side of shorter packet length as theindicator SDI becomes smaller ((A)→(E)). In other words, it isunderstood that, as the indicator SDI becomes smaller, the optimalpacket length becomes shorter. To determine a position of such a minimumpoint, the expected value <T(k)> of the total time represented by theabove formula (6) is differentiated by a variable k, and when thedifferential value (formula (7) below) becomes “0”, the packet length kat the time is selected. As a result, the optimal packet length isdetermined by the arithmetic formula (8) below.

$\begin{matrix}{\frac{{\langle{T(k)}\rangle}}{k} \propto {k^{2} + {\left( \frac{t_{1}}{t_{0}} \right) \cdot k} + {\left( \frac{t_{1}}{t_{0}} \right)\frac{1}{\log \left( {1 - p} \right)}}}} & (7) \\{k = {\frac{1}{2}\left( {{- \frac{t_{1}}{t_{0}}} \pm \sqrt{\left( \frac{t_{1}}{t_{0}} \right)^{2} - {\frac{4}{\log \left( {1 - p} \right)}\left( \frac{t_{1}}{t_{0}} \right)}}} \right)}} & (8)\end{matrix}$

Based on the above formula (8), a result calculated by using the sametimes t0 and t1 as those of the example in FIG. 12 is shown in tabularform in FIG. 13. The table in FIG. 13 is shown in graph form in FIG. 14.By calculating the packet length by the calculations described above, asillustrated in FIG. 13, the packet length is determined with highaccuracy in accordance with the value of the indicator SDI. As shown inFIG. 14, since the optimal packet length smoothly changes as theindicator SDI changes, when employing this method, there is littledrastic change in the packet length. Such a configuration can berealized by storing information of the arithmetic formula represented bythe above formula (8) in the packet length calculation section 236.

So far, the configurations of the communication system 10 according tothe first embodiment of the present invention, the first communicationapparatus 100, and the second communication apparatus 200 including themodified example have been described in detail. By applying the aboveconfigurations, the communication condition of the transmission path canbe appropriately evaluated, and the packet length can be dynamicallymodified in accordance with the evaluation result.

2: Second Embodiment

Next, the second embodiment of the present invention will be described.In the first embodiment described above, it is assumed that thecommunication conditions of the communication channels CH12 and CH21 arethe same. Therefore, the indicator SDI is calculated based on themodulated signal transmitted from the first communication apparatus 100to the second communication apparatus 200, and the indicator SDI is usedas a base for determining the packet length of the packet transmittedfrom the second communication apparatus 200 to the first communicationapparatus 100. However, the indicator SDI represents the communicationcondition of the communication channel CH12. Therefore, when thecommunication conditions of the communication channels CH12 and CH21 aredifferent, the indicator SDI is not an appropriate base.

Accordingly, in the second embodiment of the present invention, a methodis proposed in which an appropriate communication condition is evaluatedand the packet length is modified in accordance with the evaluationresult when the communication conditions of the communication channelsCH12 and CH21 are different from each other.

2-1: Packet Length Modification Method

First, an entire process flow including the packet length modificationprocessing according to this embodiment will be described with referenceto FIG. 15. FIG. 15 is an illustration showing the entire process flowincluding the packet length modification processing according to thisembodiment. Each step shown in FIG. 15 is realized by the firstcommunication apparatus 300 or the second communication apparatus 400included in the communication system 20 described below.

As shown in FIG. 15, first, data is transmitted from first communicationapparatus 300 to the second communication apparatus 400 (S202). The datatransmitted in step S202 is arbitrary data. The data to be transmittedis modulated by a predetermined modulation method, and transmitted tothe second communication apparatus 400 as a modulated signal. Next, thesecond communication apparatus 400 receives the data (modulated signal)transmitted from the first communication apparatus 300 (S204). Next, thesecond communication apparatus 400 generates a soft decision data stringfrom the received modulated signal (S206). The soft decision data stringcan be detected, for example, by using an analog-digital converter or asoft decision data detection method described below.

Next, the second communication apparatus 400 performs predeterminedstatistical processing using the soft decision data string generated instep S206, and calculates an indicator for evaluating the communicationcondition in the communication channel CH12 (S208). This indicatorindicates a degree of variation in the soft decision data stringgenerated in step S206. As described above, an evaluation indicatorcorresponding to the transmission error rate can be calculated based onthe degree of variation (for example, variance value) in the softdecision data. In step S208, such an evaluation indicator is calculated.For example, an indicator which becomes a large value when thecommunication condition of the communication channel CH12 is bad, andbecomes a small value when the communication condition is good iscalculated.

Next, the second communication apparatus 400 transmits the indicatorcalculated in step S208 to the first communication apparatus 300 (S210).Next, the first communication apparatus 300 receives the indicatortransmitted from the second communication apparatus 400 (S212). Next,the first communication apparatus 300 modifies the packet length of thedata series to be transmitted based on the indicator received in stepS212 (S214). For example, the first communication apparatus 300 performsa modification to shorten the packet length when the value of theindicator is large, and performs a modification to lengthen the packetlength when the value of the indicator is small. At this time, theindicator used by the first communication apparatus 300 to modify thepacket length is an evaluation result of the communication condition ofthe communication channel CH12.

Therefore, an appropriate packet length is determined considering thecommunication condition of the communication channel CH12. Next, thefirst communication apparatus 300 divides the data by the modifiedpacket length, and transmits the data to the second communicationapparatus 400 by packetizing each divided data (S216). The packet ofeach divided data (hereinafter, divided packet) is modulated by apredetermined modulation method, and transmitted as a modulated signal.Next, the data of the divided packet is received by the secondcommunication apparatus 400 (S218). When receiving all the dividepackets, the second communication apparatus 400 combines the dividedpackets to restore the original data.

By using such a method, even when the communication conditions of thecommunication channels CH12 and CH21 are different, the packet length inthe transmission system can be dynamically modified in accordance withthe transmission path condition. Further, even when channelcharacteristics of the communication channel CH12 change, data can bedividedly transmitted by an appropriate packet length, so that thecommunication time is shortened. As a result, the convenience of theuser can be improved.

Regarding the packet length modification method according to thisembodiment, the general process flow has been described. As describedabove, even when the channel characteristics of the communicationchannels CH12 and CH21 are different, the technique described for theabove first embodiment can be easily used. Next, functionalconfigurations of each communication apparatus for realizing the abovemethod will be described.

2-2: Functional Configuration of Communication Apparatus

Hereinafter, functional configurations of the first communicationapparatus 300 and the second communication apparatus 400 included in thecommunication system 20 which can realize the above method will bedescribed in detail. The technical features of this embodiment areincluded in both of the first communication apparatus 300 and the secondcommunication apparatus 400.

2-2-1: Functional Configuration of First Communication Apparatus 300

First, a functional configuration of the first communication apparatus300 will be described with reference to FIG. 16. FIG. 16 shows afunctional configuration example of the first communication apparatus300 according to this embodiment.

As shown in FIG. 16, the first communication apparatus 300 mainlyincludes a signal reception section 302, a demodulation section 304, anindicator extraction section 306, a communication control section 308, apacket length modification section 310, a modulation section 312, and asignal transmission section 314. As described above, a characteristicfunction of the first communication apparatus 300 according to thisembodiment is to transmit a signal to the second communication apparatus400, receive an indicator calculated based on the signal from the secondcommunication apparatus 400, and modify the packet length.

First, a packet of transmission data is input into the modulationsection 312 via the packet length modification section 310. The packetinput here may be a divided packet or an undivided packet. When thepacket is input, in the modulation section 312, the packet is modulatedbased on a predetermined modulation method, and a modulated signal isgenerated. As the predetermined modulation method, for example, thephase shift keying method, the amplitude modulation method, and the likeare used. The modulated signal generated by the modulation section 312is input into the signal transmission section 314. When the modulatedsignal is input, the signal transmission section 314 transmits themodulated signal to the second communication apparatus 400.

As described above, in the second communication apparatus 400, theindicator is calculated based on the received modulated signal, and theindicator is transmitted to the first communication apparatus 300 inreturn. The first communication apparatus 300 receives the modulatedsignal including the indicator by the signal reception section 302. Themodulated signal received by the signal reception section 302 is inputinto the demodulation section 304. In the demodulation section 304, themodulated signal is demodulated, and a data portion of the packetincluded in the modulated signal is output. The received data outputfrom the demodulation section 304 is input into the indicator extractionsection 306.

When the received data is input, the indicator extraction section 306reads the indicator from the received data. The indicator read by theindicator extraction section 306 is input into the communication controlsection 308 and the packet length modification section 310. When theindicator is input, the communication control section 308 stops datatransmission or re-starts the stopped data transmission in accordancewith the value of the indicator. For example, when the indicator isgreater than a predetermined value, the communication control section308 determines that the communication environment deteriorates, andinputs a control signal for stopping the data transmission into thesignal transmission section 314. On the other hand, when the indicatoris smaller than a predetermined value, the communication control section308 determines that the communication environment improves, and inputs acontrol signal for re-starting the data transmission into the signaltransmission section 314.

The above predetermined values are indicator values for coping with acommunication condition in which, even when modifying the packet length,data is difficult to transmit in a predetermined time period. By makingsuch a configuration, it is possible to stop the data transmission whenthe communication environment deteriorates, and re-start the datatransmission after a random or a predetermined time has passed, or afterthe communication environment has improved. When there are manycommunication devices nearby which use the same channel, datatransmission timing may need to be appropriately adjusted. In such asituation, when each communication device calculates the aboveindicator, and controls transmission timing based on the indicator, thedata transmission timing can be adjusted more efficiently.

When the indicator is input, the packet length modification section 310modifies the packet length in accordance with the value of theindicator, and generates a divided packet by the modified packet length.

For example, the packet length modification section 310 includes apacket length selection section 332 and a packetizing section 334 asshown in part (A) of FIG. 17. The packet length selection section 332 isa means for selecting an appropriate packet length in accordance withthe input indicator. The packet length selection section 332 refers tothe input indicator and the indicator table (refer to FIG. 10) in whichthe indicators and predetermined packet lengths are associated with eachother, and selects an appropriate packet length. The packet lengthinformation selected by the packet length selection section 332 is inputinto the packetizing section 334. When the packet length information isinput, the packetizing section 334 divides transmission data by thepacket length selected by the packet length selection section 332, andgenerates a divided packet by packetizing the divided transmission data.

For another example, the packet length modification section 310 includesa packet length calculation section 336 and the packetizing section 334as shown in part (B) of FIG. 17. The packet length calculation section336 is a means for calculating an appropriate packet length inaccordance with the input indicator. For example, the packet lengthcalculation section 336 holds information of the arithmetic formularepresented by the above formula (8), and calculates a packet lengthadapted to the indicator by using the arithmetic formula. The packetlength information calculated by the packet length calculation section336 is input into the packetizing section 334. When the packet lengthinformation is input, the packetizing section 334 divides transmissiondata by the packet length calculated by the packet length calculationsection 336, and generates a divided packet by packetizing the dividedtransmission data.

The divided packet generated by the packet length modification section310 in that way is input into the modulation section 312. When thedivided packet is input from the packet length modification section 310,the modulation section 312 modulates the divided packet based on apredetermined modulation method, and generates a modulated signal. Themodulated signal generated by the modulation section 312 is input intothe signal transmission section 314. When the modulated signal is input,the signal transmission section 314 transmits the input modulated signalto the second communication apparatus 400. The signal transmissionsection 314 stops the transmission of the modulated signal or re-startsthe transmission in accordance with the control signal input from thecommunication control section 308. The signal transmission section 314may be configured to transmit a test packet for checking thecommunication condition in the transmission path to the secondcommunication apparatus 400 when the control signal to stop thetransmission is input from the communication control section 308.

So far, the functional configuration of first communication apparatus300 has been described.

2-2-2: Functional Configuration of Second Communication Apparatus 400

Next, a functional configuration of the second communication apparatus400 will be described with reference to FIG. 16. FIG. 16 shows afunctional configuration example of the second communication apparatus400 according to this embodiment.

As shown in FIG. 16, the second communication apparatus 400 mainlyincludes a signal reception section 402, a soft decision data generationsection 404, an indicator calculation section 406, a packetizing section408, a modulation section 410, a signal transmission section 412, and ademodulation section 410.

As described above, when the modulated signal is transmitted from thefirst communication apparatus 300, the modulated signal is received bythe signal reception section 402. The modulated signal received by thesignal reception section 402 is input into the soft decision datageneration section 404. When the modulated signal is input, the softdecision data generation section 404 generates soft decision data fromthe modulated signal. The soft decision data generation section 404detects each bit value as a value having two or more bits (soft decisiondata value) by using, for example, analog-digital conversion or a softdecision data detection method (refer to FIGS. 27, 28, and 29) describedbelow. For example, the soft decision data generation section 404 candetect a soft decision data string having amplitude values as shown inFIG. 4.

The soft decision data string generated by the soft decision datageneration section 404 is input into the indicator calculation section406. When the soft decision data string is input, the indicatorcalculation section 406 counts the number of data values (parameterERNUM) included in the soft decision data string. The indicatorcalculation section 406 also calculates absolute values of each softdecision data. The indicator calculation section 406 sums up theabsolute values of each soft decision data and calculates an absolutevalue sum (ERABS). Further, the indicator calculation section 406calculates an average of the absolute values (absolute value averageAVE) by using the parameter ERNUM and the absolute value sum ERABS.Also, the indicator calculation section 406 calculates square values ofeach soft decision data and calculates the sum of the square values(square value sum ERSQA).

The indicator calculation section 406 calculates a standard deviation(STDEV) of the soft decision data string by using the parameter ERNUM,the square value sum ERSQA, and the absolute value sum ERABS. Theabsolute value average AVE and the standard deviation STDEV arecalculated based on the above formulas (1) and (2). When the absolutevalue average AVE and the standard deviation STDEV are calculated, theindicator calculation section 406 calculates the indicator (SDI) basedon the above formula (3). The indicator SDI calculated by the indicatorcalculation section 406 is input into the packetizing section 408.Transmission data to be transmitted to the first communication apparatus300 is also input into the packetizing section 408.

When the indicator SDI is input, the packetizing section 408 packetizesthe indicator SDI along with the transmission data and generates atransmission packet (refer to FIG. 18). Here, a configuration of thetransmission packet generated by the packetizing section 408 will bebriefly described with reference to FIG. 18. FIG. 18 is an illustrationshowing the packet configuration including the indicator SDI and anindicator transmission method. FIG. 18 shows a configuration of atransmission packet transmitted from the first communication apparatus300 to the second communication apparatus 400 (hereinafter, transmissionpacket P12) and a configuration of a transmission packet transmittedfrom the second communication apparatus 400 to the first communicationapparatus 300 (hereinafter, transmission packet P21).

First, a configuration of the transmission packet P12 will be described.The transmission packet P12 includes a preamble (preamble), asynchronous code (sync), a length (length), a length check (Lcheck),data (data), and an error check (CRC). Further, the data (data) mayinclude a command (command) as shown in the example of FIG. 18. In thiscommand (command), for example, a control command for controlling theoperation of the second communication apparatus 400. As described above,when receiving the transmission packet P12, the second communicationapparatus 400 calculates the indicator SDI by the indicator calculationsection 406. At this time, the indicator calculation section 406calculates the indicator SDI by using the synchronous code sync andfollowing data.

On the other hand, the transmission packet P21 includes a preamble(preamble), a synchronous code (sync), a length (length), a length check(Lcheck), data (data), and an error check (CRC). However, the data(data) in the transmission packet P21 includes a response (response), anindicator (indicator; ERNUM, ERABS, ERSQA), and data “data”. In otherwords, in the transmission packet P21, the indicator (indicator) isinserted in the data (data) portion. This insertion processing isperformed by the packetizing section 408. In the first communicationapparatus 300, the indicator (indicator) included in the data (data) isextracted by the indicator extraction section 306. The response(response) described above represents response data to the command(command).

The transmission packet P21 in which the indicator SDI is inserted inthat way is input from the packetizing section 408 into the modulationsection 410. When the transmission packet P21 is input from thepacketizing section 408, the modulation section 410 modulates thetransmission packet P21 based on a predetermined modulation method, andgenerates a modulated signal. The modulated signal generated by themodulation section 410 is input into the signal transmission section412. When the modulated signal is input, the signal transmission section412 transmits the input modulated signal to the first communicationapparatus 300. As described above, the first communication apparatus 300extracts the indicator SDI from the received modulated signal, anddetermines an appropriate packet length based on the indicator SDI.Then, the first communication apparatus 300 divides data by thedetermined packet length, and transmits the divided packet to the secondcommunication apparatus 400.

The second communication apparatus 400 receives the modulated signalincluding the divided packet by the signal reception section 402. Themodulated signal, which corresponds to each divided packet, received bythe signal reception section 402 is input into the demodulation section414. When the modulated signal is input, the demodulation section 414demodulates the modulated signal, and extracts divided data from eachdivided packet. When extracting all the divided data, the demodulationsection 414 combines the divided data to restore the original data. Thedata restored by the demodulation section 414 in that way is output asreception data. The modulated signal received by the signal receptionsection 402 is also input into the soft decision data generation section404, and used to calculate the indicator SDI. The processing such astransmitting the indicator SDI and modifying the packet length isrepeatedly performed.

So far, the functional configuration of the second communicationapparatus 400 has been described. By using such a method, even when thecommunication conditions of the communication channels CH12 and CH21 aredifferent, the packet length in the transmission system can bedynamically modified in accordance with the transmission path condition.Further, even when channel characteristics of the communication channelCH12 change, data can be dividedly transmitted by an appropriate packetlength, so that the communication time is shortened. As a result, theconvenience of the user can be improved.

So far, the configurations of the communication system 20 according tothe second embodiment of the present invention, the first communicationapparatus 300, and the second communication apparatus 400 have beendescribed in detail while comparing with the contents of the above firstembodiment. By applying the above configurations, the communicationcondition of the transmission path can be appropriately evaluated, andthe packet length can be dynamically modified in accordance with theevaluation result.

3: Third Embodiment

Next, the third embodiment of the present invention will be described.In the second embodiment described above, a method in which onecommunication channel CH12 is evaluated and the packet length ismodified in accordance with the evaluation result is shown. Thisembodiment evaluates both of the communication channels CH12 and CH21,and improves the two-way communication environment by using theevaluation result. However, note that basic technical idea is derivedfrom the second embodiment described above.

3-1: Functional Configuration of Communication Apparatus

First, functional configurations of a first communication apparatus 500and a second communication apparatus 600 included in a communicationsystem 30 according to this embodiment will be described in detail.

3-1-1: Functional Configuration of First Communication Apparatus 500

First, a functional configuration of the first communication apparatus500 will be described with reference to FIG. 19. FIG. 19 shows afunctional configuration example of the first communication apparatus500 according to this embodiment.

As shown in FIG. 19, the first communication apparatus 500 mainlyincludes a signal reception section 502, a soft decision data generationsection 504, an indicator calculation section 506, a demodulationsection 508, and an indicator extraction section 510. Further, the firstcommunication apparatus 500 includes a communication control section512, a packet length modification section 514, a modulation section 516,and a signal transmission section 518.

First, a packet of transmission data is input into the modulationsection 516 via the packet length modification section 514. The packetinput here may be a divided packet or an undivided packet. When thepacket is input, in the modulation section 516, the packet is modulatedbased on a predetermined modulation method, and a modulated signal isgenerated. As the predetermined modulation method, for example, thephase shift keying method, the amplitude modulation method, and the likeare used. The modulated signal generated by the modulation section 516is input into the signal transmission section 518. When the modulatedsignal is input, the signal transmission section 518 transmits themodulated signal to the second communication apparatus 600.

In the second communication apparatus 600, an indicator is calculatedbased on the received modulated signal, and the indicator is transmittedto the first communication apparatus 500 in return. The firstcommunication apparatus 500 receives the modulated signal including theindicator by the signal reception section 502. The modulated signalreceived by the signal reception section 502 is input into the softdecision data generation section 504 and the demodulation section 508.When the modulated signal is input, the soft decision data generationsection 504 generates soft decision data from the modulated signal. Thesoft decision data generation section 504 detects each bit value as avalue having two or more bits (soft decision data value) by using, forexample, analog-digital conversion or a soft decision data detectionmethod (refer to FIGS. 27, 28, and 29) described below. For example, thesoft decision data generation section 504 can detect a soft decisiondata string having amplitude values as shown in FIG. 4.

The soft decision data string generated by the soft decision datageneration section 504 is input into the indicator calculation section506. When the soft decision data string is input, the indicatorcalculation section 506 counts the number of data values (parameterERNUM) included in the soft decision data string. The indicatorcalculation section 506 also calculates absolute values of each softdecision data. The indicator calculation section 506 sums up theabsolute values of each soft decision data and calculates an absolutevalue sum (ERABS). Further, the indicator calculation section 506calculates an average of the absolute values (absolute value averageAVE) by using the parameter ERNUM and the absolute value sum ERABS.Also, the indicator calculation section 506 calculates square values ofeach soft decision data and calculates the sum of the square values(square value sum ERSQA).

The indicator calculation section 506 calculates a standard deviation(STDEV) of the soft decision data string by using the parameter ERNUM,the square value sum ERSQA, and the absolute value sum ERABS. Theabsolute value average AVE and the standard deviation STDEV arecalculated based on the above formulas (1) and (2). When the absolutevalue average AVE and the standard deviation STDEV are calculated, theindicator calculation section 506 calculates the indicator (SDI) basedon the above formula (3). The indicator SDI calculated by the indicatorcalculation section 506 is input into the packet length modificationsection 514.

On the other hand, in the demodulation section 508, the modulated signalis demodulated, and a data portion of the packet included in themodulated signal is output. The received data output from thedemodulation section 508 is input into the indicator extraction section510. When the received data is input, the indicator extraction section510 reads the indicator from the received data. The indicator read bythe indicator extraction section 510 is input into the communicationcontrol section 512 and the packet length modification section 514.

When the indicator is input, the communication control section 512 stopsdata transmission or re-starts the stopped data transmission inaccordance with the value of the indicator. For example, when theindicator is greater than a predetermined value, the communicationcontrol section 512 determines that the communication environmentdeteriorates, and inputs a control signal for stopping the datatransmission into the signal transmission section 518. On the otherhand, when the indicator is smaller than a predetermined value, thecommunication control section 512 determines that the communicationenvironment improves, and inputs a control signal for re-starting thedata transmission into the signal transmission section 518.

The above predetermined values are indicator values for coping with acommunication condition in which, even when modifying the packet length,data is difficult to transmit in a predetermined time period. By makingsuch a configuration, it is possible to stop the data transmission whenthe communication environment deteriorates, and re-start the datatransmission after a random or a predetermined time has passed, or afterthe communication environment has improved. When there are manycommunication devices nearby which use the same channel, datatransmission timing may need to be appropriately adjusted. In such asituation, when each communication device calculates the aboveindicator, and controls transmission timing based on the indicator, thedata transmission timing can be adjusted more efficiently.

When the indicator from the indicator extraction section 510 is input,the packet length modification section 514 modifies the packet length inaccordance with the value of the indicator, and generates a dividedpacket by the modified packet length. However, the packet lengthmodification section 514 inserts the indicator SDI calculated by theindicator calculation section 506 into the data (data) portion of thedivided packet as shown in part (B) of FIG. 18. The divided packetgenerated by the packet length modification section 514 in that way isinput into the modulation section 516.

When the divided packet is input from the packet length modificationsection 514, the modulation section 516 modulates the divided packetbased on a predetermined modulation method, and generates a modulatedsignal. The modulated signal generated by the modulation section 516 isinput into the signal transmission section 518. When the modulatedsignal is input, the signal transmission section 518 transmits the inputmodulated signal to the second communication apparatus 600. The signaltransmission section 518 stops the transmission of the modulated signalor re-starts the transmission in accordance with the control signalinput from the communication control section 512. The signaltransmission section 518 may be configured to transmit a test packet forchecking the communication condition in the transmission path to thesecond communication apparatus 600 when the control signal to stop thetransmission is input from the communication control section 512.

So far, the functional configuration of the first communicationapparatus 500 has been described.

3-1-2: Functional Configuration of Second Communication Apparatus 600

Next, a functional configuration of the second communication apparatus600 will be described with reference to FIG. 19. FIG. 19 shows afunctional configuration example of the second communication apparatus600 according to this embodiment. However, the configuration of thesecond communication apparatus 600 is substantially the same as that ofthe first communication apparatus 500. Therefore, detailed explanationof constituent elements is omitted, and only correspondencerelationships between each constituent element will be described.

As shown in FIG. 19, the second communication apparatus 600 mainlyincludes a signal reception section 602, a soft decision data generationsection 604, an indicator calculation section 606, a demodulationsection 608, and an indicator extraction section 610. The signalreception section 502, the soft decision data generation section 504,the indicator calculation section 506, the demodulation section 508, andthe indicator extraction section 510 included in the first communicationapparatus 500 correspond to the above constituent elements respectively.Further, the second communication apparatus 600 includes a communicationcontrol section 612, a packet length modification section 614, amodulation section 616, and a signal transmission section 618. Thecommunication control section 512, the packet length modificationsection 514, the modulation section 516, and the signal transmissionsection 518 included in the first communication apparatus 500 correspondto the above constituent elements respectively. The flow of data andsignal transmitted between each constituent element is the same as thatof the first communication apparatus 500.

So far, the functional configuration of the second communicationapparatus 600 has been described.

By applying such a configuration, even when the communication conditionsof the communication channels CH12 and CH21 are different, the packetlength in the transmission system can be dynamically modified inaccordance with the transmission path condition. Further, even whenchannel characteristics of the communication channels CH12 and CH 21change, data can be dividedly transmitted in two-way by an appropriatepacket length, so that the communication time is shortened. As a result,the convenience of the user can be improved.

3-2: Hardware Configuration of Each Communication Apparatus

Next, a hardware configuration of the first communication apparatus 500and the second communication apparatus 600 will be described withreference to FIGS. 20 to 23. Since the first communication apparatus 500and the second communication apparatus 600 have substantially the sameconfiguration, the description of the second communication apparatus 600is omitted.

First, refer to FIG. 20. As shown in FIG. 20, the function of the firstcommunication apparatus 500 is realized by including a communicationsection H52, an indicator register H54, an arithmetic processing sectionH56 (CPU), and a display apparatus H58. The indicator register H54 isshared between the communication section H52 and the arithmeticprocessing section H56. A keyboard and various switches are additionallyprovided to the first communication apparatus 500. Further, the displayapparatus H58 on which the indicator is displayed may be omitted. Thesecond communication apparatus 600 has a similar configuration to thatdescribed above.

(Register Configuration: Communication Section H52 and IndicatorRegister H54)

Next, refer to FIG. 21. FIG. 21 shows a register group included in thecommunication section H52, and indicator register H54. First, wheninformation indicating a start of reception processing is input in atransmission/reception control register 710, a reception control section706 starts the reception processing. When the reception processing isstarted and an analog reception signal is input into a wave detectionsection 702, the result detected by the wave detection section 702 isinput into a synchronization determination section 704. In thesynchronization determination section 704, data is detected by using ananalog-digital converter or the like, and a soft decision data string isgenerated as detected data.

Hard decision data is generated from the soft decision data generated inthe synchronization determination section 704, and input into thereception control section 706. The reception control section 706 storesthe input hard decision data into a reception data register 714. Thereception control section 706 stores information (OK or NG) indicatingthe reception condition into the reception data register 714 inaccordance with the reception condition. The reception control section706 stores information indicating the end of the reception processinginto the transmission/reception control register 710. When theinformation indicating the end of the reception processing is stored inthe transmission/reception control register 710, the arithmeticprocessing section 1156 refers to the information and recognizes the endof the reception processing.

On the other hand, the soft decision data output from thesynchronization determination section 704 is input into a statisticalprocessing section 708. As described below, the statistical processingsection 708 calculates the parameter ERNUM, the absolute value sumERABS, and the square value sum ERSQA from the soft decision data. Theparameter ERNUM, the absolute value sum ERABS, and the square value sumERSQA calculated by the statistical processing section 708 are stored inthe indicator register 1154. The parameter ERNUM, the absolute value sumERABS, and the square value sum ERSQA stored in the indicator registerH54 are read by the arithmetic processing section 1156, and used tocalculate the indicator SDI.

When information indicating a start of transmission processing is inputin the transmission/reception control register 710, a transmissioncontrol section 718 starts the transmission processing. The transmissioncontrol section 718 reads the transmission data stored in a transmissiondata register 712, and inputs the transmission data into a modulationsection 720. The transmission data series input into the modulationsection 720 is modulated by a predetermined modulation method, andtransmitted as an analog transmission signal. When the transmissionprocessing ends, the transmission control section 718 stores informationindicating the end of the transmission processing into thetransmission/reception control register 710. The arithmetic processingsection H56 refers to the information stored in thetransmission/reception control register 710, and recognizes the end ofthe transmission processing.

The transmission/reception processing is realized by the registerconfiguration as described above.

(Details of Statistical Processing Section 708)

Next, refer to FIG. 22. In FIG. 22, a circuit configuration of thestatistical processing section 708 is described. The statisticalprocessing section 708 is constituted by a block for obtaining aparameter NUM, a block for obtaining an absolute value sum AVE, and ablock for obtaining a square value sum VAR.

(Block for Obtaining Parameter NUM)

The block for obtaining the parameter NUM includes an adder 722, an ANDcircuit 724, and D flip-flops 726, 728. This block functions as acounter.

First, a bit value “1” is input into the adder 722 every time the softdecision data is input. The adder 722 adds the input bit value “1” and atemporary parameter NUM-tmp input from the D flip-flop 726. The newtemporary parameter NUM-tmp output from the adder 722 is input into theAND circuit 724.

A reset signal is input into the AND circuit 724. When the reset signalis not input into the AND circuit 724, the AND circuit 724 inputs avalue indicating the temporary parameter NUM-temp input from the adder722 into the D flip-flop 726. When the reset signal is input into theAND circuit 724, the AND circuit 724 inverts the reset signal, obtainsthe inverted reset signal, and inputs a bit value “0” into the Dflip-flop 726 as the temporary parameter NUM-temp. In other words, theAND circuit 724 resets the temporary parameter NUM-tmp to 0 at thetiming when the reset signal is input.

The D flip-flop 726 obtains the value indicating the temporary parameterNUM-temp from the AND circuit 724 at a timing when an input data clockDClk rises. The D flip-flop 726 holds the obtained value until the nextdata clock DClk rises. The D flip-flop 726 inputs the temporaryparameter NUM-temp, which the D flip-flop 726 holds, into the adder 722and the D flip-flop 728 at the timing when the data clock DClk rises. Inother words, the D flip-flop 726 updates the temporary parameterNUM-temp, which the D flip-flop 726 holds, at the timing when the dataclock DClk rises.

The reset signal is input into the D flip-flop 728 in the same way asinto the AND circuit 724. When the reset signal is input into the Dflip-flop 728, the D flip-flop 728 obtains the temporary parameterNUM-temp from the D flip-flop 726, and outputs the obtained value as thefinal parameter NUM.

(Block for Obtaining Absolute Value Sum Ave)

The block for obtaining the absolute value sum AVE includes an absolutevalue circuit 730, an adder 732, an AND circuit 734, and D flip-flops736, 738.

The absolute value circuit 730 calculates an absolute value of the inputsoft decision data. The calculation result output from the absolutevalue circuit 730 is input into the adder 732. The adder 732 adds theabsolute value of the soft decision data input from the absolute valuecircuit 730 and a temporary absolute value sum AVE-tmp input from the Dflip-flop 736. The new temporary absolute value sum AVE-tmp output fromthe adder 732 is input into the AND circuit 734.

The reset signal is input into the AND circuit 734. When the resetsignal is not input into the AND circuit 734, the AND circuit 732 inputsthe temporary absolute value sum AVE-tmp input from the adder 732 intothe D flip-flop 736. When the reset signal is input into the AND circuit734, the AND circuit 734 inverts the reset signal, obtains the invertedreset signal, and inputs a bit value “0” into the D flip-flop 736 as thetemporary absolute value sum AVE-tmp. In other words, the AND circuit734 resets the temporary absolute value sum AVE-tmp to 0 at the timingwhen the reset signal is input.

The D flip-flop 736 obtains the temporary absolute value sum AVE-tmpfrom the AND circuit 734 at the timing when the input data clock DClkrises. The D flip-flop 736 holds the obtained value until the next dataclock DClk rises. The D flip-flop 736 inputs the temporary absolutevalue sum AVE-tmp, which the D flip-flop 736 holds, into the adder 732and the D flip-flop 738 at the timing when the data clock DClk rises. Inother words, the D flip-flop 736 updates the temporary absolute valuesum AVE-tmp, which the D flip-flop 736 holds, at the timing when thedata clock DClk rises.

The reset signal is input into the D flip-flop 738 in the same way asinto the AND circuit 734. When the reset signal is input into the Dflip-flop 738, the D flip-flop 738 obtains a value indicating thetemporary absolute value sum AVE-tmp from the D flip-flop 736, andoutputs the obtained value as a value indicating the final absolutevalue sum AVE.

(Block for Obtaining Square Value Sum VAR)

The block for obtaining the square value sum VAR includes a square valuecircuit 740, an adder 742, an AND circuit 744, and D flip-flops 746,748.

The square value circuit 740 squares the value of the soft decision datainput from the adder 223 and outputs the square value. The square valueoutput from the square value circuit 740 is input into the adder 742.The adder 742 adds the square value of the soft decision data input fromthe square value circuit 740 and a temporary square value sum VAR-tmpinput from the D flip-flop 746. The new temporary square value sumVAR-tmp output from the adder 742 is input into the AND circuit 744.

The reset signal is input into the AND circuit 744. When the resetsignal is not input into the AND circuit 744, the AND circuit 744 inputsthe temporary square value sum VAR-tmp input from the adder 742 into theD flip-flop 746. When the reset signal is input into the AND circuit744, the AND circuit inverts the reset signal, obtains the invertedreset signal, and inputs a bit value “0” into the D flip-flop 746 as thetemporary square value sum VAR-tmp. In other words, the AND circuit 744resets the temporary square value sum VAR-tmp to 0 at the timing whenthe reset signal is input.

The D flip-flop 746 obtains the temporary square value sum VAR-tmp fromthe AND circuit 744 at the timing when the input data clock DClk rises.The D flip-flop 746 holds the obtained value until the next data clockDClk rises. The D flip-flop 746 inputs the temporary square value sumVAR-tmp, which the D flip-flop 746 holds, into the adder 742 and the Dflip-flop 748 at the timing when the data clock DClk rises. In otherwords, the D flip-flop 746 updates the temporary square value sumVAR-tmp, which the D flip-flop 746 holds, at the timing when the dataclock DClk rises.

The reset signal is input into the D flip-flop 748 in the same way asinto the AND circuit 748. When the reset signal is input into the Dflip-flop 748, the D flip-flop 748 obtains a value indicating thetemporary square value sum VAR-tmp from the D flip-flop 746, and outputsthe obtained value as the final square value sum VAR. The timing whenthe reset signal is input into the AND circuits 724, 734, and 744 iswhen the temporary parameter NUM-temp reaches a predetermined number.This timing may be when the detection of the soft decision data ends.

For example, in the case when the reset signal is input at the timingwhen the detection of the soft decision data ends, when the detection ofthe soft decision data starts at the same time as when the reception ofthe reception signal starts, the level of the reset signal is set to Low(value “0”). On the other hand, when the detection of the soft decisiondata ends at the same time as when the reception of the reception signalends, the level of the reset signal is set to High (value “1”). The Dflip-flop 728 outputs the parameter NUM at the timing when the resetsignal goes High. The parameter NUM, the absolute value sum AVE, and thesquare value sum VAR may be configured to be updated every time the softdecision data obtained from the reception signal of one packet isprocessed instead of using the temporary parameter NUM-temp.

As described above, in the statistical processing section 708, everytime the reset signal is input, the parameter NUM (ERNUM describedabove), the absolute value sum AVE (ERABS described above), and thesquare value sum VAR (ERSQA described above) are updated. The updatedvalues are stored in the indicator register H54.

(Details of Arithmetic Processing Section H56)

Next, refer to FIG. 23. In FIG. 23, a configuration example of thearithmetic processing section H56 is described. As shown in FIG. 23, thearithmetic processing section H56 includes an indicatorcalculation/packet length selection section 752, an indicator table 754,a transmission data generation section 756, and a transmission datamemory 758.

When receiving an interrupt signal indicating the end of reception fromthe communication section H52, the arithmetic processing section H56reads the received data and a flag indicating the reception conditionstored in the register group in the communication section H52. At thistime, the arithmetic processing section H56 also reads sequentially theparameter (ERNUM), the absolute value sum (ERABS), and the square valuesum (ERSQA) stored in the indicator register H54. The parameter (ERNUM),the absolute value sum (ERABS), and the square value sum (ERSQA) readfrom the indicator register H54 are input into the indicatorcalculation/packet length selection section 752.

The indicator calculation/packet length selection section 752 calculatesthe indicator SDI by using the input parameter ERNUM, absolute value sumERABS, and square value sum ERSQA. Further, the indicatorcalculation/packet length selection section 752 refers to the indicatortable 754 (refer to FIG. 10), and selects an packet length correspondingto the calculated indicator SDI. In other words, the indicatorcalculation/packet length selection section 752 corresponds to the abovedescribed indicator calculation section 506 and packet lengthmodification section 514. The packet length selected by the indicatorcalculation/packet length selection section 752 is transmitted to thetransmission data generation section 756.

The transmission data generation section 756 reads the transmission datastored in the transmission data memory 758, divides the transmissiondata so that the length of the transmission data suits the transmittedpacket length, and generates a divided packet. Then, the transmissiondata generation section 756 stores the divided packet into thetransmission data register 712 and stores a flag indicating the start oftransmission into the transmission/reception control register 710. Inother words, the transmission data generation section 756 corresponds toa part of the function of the packet length modification section 514.

So far, the hardware configuration of the first communication apparatus500 has been described. The functional configuration of the firstcommunication apparatus 500 as shown in FIG. 19 is realized by theconfiguration described above. Of course, the second communicationapparatus 600 has a similar configuration to that described above.Therefore, it is possible to appropriately control the packet lengthaccording to the communication environment, and shorten the totalcommunication time, so that the convenience of the user improvessignificantly.

3-3: (Modified Example) Hardware Configuration of Each CommunicationApparatus

Next, a modified example of the above hardware configuration will bebriefly described with reference to FIG. 24. A configuration in which acontroller 540 is separately provided and a part of the function such asthe display apparatus H58 is mounted on the controller 540 isconsidered, such as the first communication apparatus 530 shown in FIG.24. For example, a contactless IC card reader/writer (so-calledreader/writer) substantially has the same configuration as that of thefirst communication apparatus 530. In this case, the first communicationapparatus 530 reciprocally transmits information via the arithmeticprocessing sections H56A and H56B. On the other hand, the secondcommunication apparatus 630 corresponds to a contactless IC card, amobile phone including a contactless IC chip, and the like. Thetechnique according to this embodiment can be applied to such aconfiguration.

3-4: Flow of Data Read/Write Processing

If the communication system 30 is constituted by a hardwareconfiguration as shown in FIG. 24, the data read/write processingincluding dynamic modification processing of the packet length will beas shown in FIG. 25. Hereinafter, the data read/write processing will bebriefly described. FIG. 25 is an illustration showing a flow of the dataread/write processing according to this embodiment.

As shown in FIG. 25, a polling command is transmitted from the firstcommunication apparatus 530 (S302). The polling command is transmittedto check whether or not the reception terminal (the second communicationapparatus 630) is present. For example, when a response to the pollingcommand is received, the first communication apparatus 530 recognizesthat the reception terminal is present. Therefore, the firstcommunication apparatus 530 determines whether or not a response to thetransmitted polling command is received from the reception terminal(S304). When receiving the response, the first communication apparatus530 proceeds to the processing of step S306. When not receiving theresponse, the first communication apparatus 530 proceeds to theprocessing of step S302 again and transmits a polling command again.

In step S306, the first communication apparatus 530 calculates theindicator in accordance with the response received from the receptionterminal, and specifies a packet length corresponding to the indicatoras the packet length used to read data (S306). Next, the firstcommunication apparatus 530 transmits a read command by specifying thepacket length specified in step S306 to the reception terminal (S308).Next, the first communication apparatus 530 determines whether or not toend the read processing (S310). When ending the read processing, thefirst communication apparatus 530 proceeds to the processing of stepS312. When continuing the read processing, the first communicationapparatus 530 proceeds to the processing of step S306 again.

When proceeding to the processing of step S312, the first communicationapparatus 530 specifies a packet length used to write data in accordancewith information of the indicator received from the reception terminal(S310). Next, the first communication apparatus 530 transmits write databy the specified packet length (S314). Next, the first communicationapparatus 530 determines whether or not to end the write processing(S316). When continuing the write processing, the first communicationapparatus 530 proceeds to the processing of step S312 again. When endingthe write processing, the first communication apparatus 530 ends theseries of the processing. Then, the first communication apparatus 530proceeds to processing for the next reception terminal.

So far, the data read/write processing according to this embodiment hasbeen described.

3-5: Transmission Method Including Dynamic Packet Length ModificationProcessing

Next, a flow of a transmission method including dynamic packet lengthmodification processing will be briefly summarized with reference toFIG. 26. As shown in FIG. 26, first, when arbitrary data is transmitted(S402), an indicator is calculated in the data destination, and whetheror not the indicator satisfies a predetermined condition is determined(S404). When the indicator satisfies the predetermined condition, theprocessing of step S406 is performed. On the other hand, when theindicator does not satisfy the predetermined condition, the firstcommunication apparatus 530 proceeds to the processing of step S402again. When the first communication apparatus 530 proceeds to theprocessing of step S406, a packet length is determined according to theindicator, and the packet length is specified to the packet length fortransmission (S406). Next, the transmission data is transmitted bydivided packets of the specified length (S408). Next, whether or not allthe data is transmitted is determined (S410). When all the data istransmitted, the first communication apparatus 530 ends the series ofthe processing, and when all the data is not yet transmitted, the firstcommunication apparatus 530 proceeds to the processing of step S406again.

By repeatedly performing the processing in the above flow, it ispossible to transmit data while dynamically modifying the packet lengthin accordance with the communication condition. As a result, the totalcommunication time used to transmit entire data is shortened, so thatthe convenience of the user improves.

4: Addendum

Hereinafter, applicable techniques in each embodiment described abovewill be briefly described.

4-1: Detection Method of Soft Decision Data

The soft decision data generation section 204 can be constituted by asampling clock generation section 800, a soft decision baseband signalgeneration section 820, and a data generation section 830. Hereinafter,circuit configuration examples of these constituent elements will bedescribed in detail. The circuit configuration examples are the same asfor the soft decision data generation sections 404, 504, and 604.

4-1-1: Circuit Configuration of Sampling Clock Generation Section 800

First, the sampling clock generation section 800 will be described. Adriving clock having a similar frequency to that of a carrier wave ofthe digitally modulated signal is input into the sampling clockgeneration section 800. This driving clock is for driving the digitalcircuit and becomes a reference to control operation timings of thedigital circuit.

The sampling clock generation section 800 generates a sampling clock bydelaying the input driving clock by a time shorter than the cycle of thedriving clock. For example, the sampling clock generation section 800generates a plurality of sampling clocks having phases different fromone another. The sampling clocks generated by the sampling clockgeneration section 800 are input into the soft decision baseband signalgeneration section 820 and the data generation section 830.

Hereinafter, a specific circuit configuration of the sampling clockgeneration section 800 will be described with reference to FIG. 27. FIG.27 is an illustration showing an example of the specific circuitconfiguration of the sampling clock generation section 800. When thedriving clock (Clock) is input, the sampling clock generation section800 illustrated in FIG. 27 generates 8 sampling clocks, phases of whichare shifted by the integral multiple of one-eighth cycle of the drivingclock.

As shown in FIG. 27, the sampling clock generation section 800 includesdelay sections 804, 808, and 812, and inverters 802, 806, 810, and 814.The function of the delay sections 804, 808, and 812 is realized by, forexample, a programmable delay element. The function of the inverters802, 806, 810, and 814 is realized by, for example, an inverter circuitoperating faster than the cycle of the driving clock.

First, the driving clock (Clock) is input. The input driving clock isoutput as a sampling clock Clk [0], and input into the delay sections804, 808, and 812, and the inverter 802. Since the sampling clock Clk[0] is the driving clock itself, the sampling clock Clk [0] has the samephase as that of the driving clock.

The inverter 802 inverts the input driving signal. The inverted drivingclock is output as a sampling clock Clk [4]. Since the sampling clockClk [4] is an inverted driving clock, the sampling clock Clk [4] has aphase delayed by four-eighth cycle from the sampling clock Clk [0].

The delay section 804 delays the driving clock by one-eighth cycle inaccordance with control signals input from a selector. The delayeddriving clock is output as a sampling clock Clk [1], and input into theinverter 806. Since the sampling clock Clk [1] is a clock delayed byone-eighth cycle from the driving clock, the sampling clock Clk [1] hasa phase delayed by one-eighth cycle from the sampling clock Clk [0].

The inverter 806 inverts the driving clock delayed by one-eighth cycle(corresponding to the sampling clock Clk [1]). The inverted drivingclock is output as a sampling clock Clk [5]. Since the sampling clockClk [5] is an inverted clock of the driving clock delayed by one-eighthcycle, the sampling clock Clk [5] has a phase delayed by five-eighthcycle from the sampling clock Clk [0].

The delay section 808 delays the driving clock by two-eighth cycle inaccordance with the control signals input from the selector. The delayeddriving clock is output as a sampling clock Clk [2], and input into theinverter 810. Since the sampling clock Clk [2] is a clock delayed bytwo-eighth cycle from the driving clock, the sampling clock Clk [2] hasa phase delayed by two-eighth cycle from the sampling clock Clk [0].

The inverter 810 inverts the driving clock delayed by two-eighth cycle(corresponding to the sampling clock Clk [2]). The inverted drivingclock is output as a sampling clock Clk [6]. Since the sampling clockClk [6] is an inverted clock of the driving clock delayed by two-eighthcycle, the sampling clock Clk [6] has a phase delayed by six-eighthcycle from the sampling clock Clk [0].

The delay section 812 delays the driving clock by three-eighth cycle inaccordance with the control signals input from the selector. The delayeddriving clock is output as a sampling clock Clk [3], and input into theinverter 814. Since the sampling clock Clk [3] is a clock delayed bythree-eighth cycle from the driving clock, the sampling clock Clk [3]has a phase delayed by three-eighth cycle from the sampling clock Clk[0].

The inverter 814 inverts the driving clock delayed by three-eighth cycle(corresponding to the sampling clock Clk [3]). The inverted drivingclock is output as a sampling clock Clk [7]. Since the sampling clockClk [7] is an inverted clock of the driving clock delayed bythree-eighth cycle, the sampling clock Clk [7] has a phase delayed byseven-eighth cycle from the sampling clock Clk [0].

In this way, eight types of sampling clocks Clk [0] to Clk [7], whichhave phases different from one another, are generated. Although, in theexample of FIG. 27, the delay sections 804, 808, 812 are configured tobe arranged in parallel, for example, a configuration in which threedelay circuits which delay the driving clock by one-eighth cycle arearranged in series may be employed. In the example of FIG. 27, since thesampling clocks are generated on the basis that the inversion changesthe phases by it, it is preferred that a signal having a duty ratio ofabout 50% is used as the driving clock.

The sampling clocks Clk [0] to Clk [7] generated in this way are inputinto the soft decision baseband signal generation section 820 and thedata generation section 830.

In the above example, the digitally modulated signal is assumed to beManchester code or a binary phase shift keying signal. For such asignal, as in the above example, it is desired that the phase of thecarrier wave is divided into eight equal parts and 8 types of samplingclocks having phases different from one another are prepared. Forexample, if the number of the types of the sampling clock is reducedfrom 8 to 4, only when the phase is shifted by one phase, the data isdifficult to be detected. Therefore, when using the Manchester code orthe binary phase shift keying signal, it is appropriate that the numberof the types of the sampling clock is about 8. When using a four-phaseshift keying signal, it is appropriate that the number of the types ofthe sampling clock is about 16.

4-1-2: Circuit Configuration of Soft Decision Baseband Signal GenerationSection 820

Next, the soft decision baseband signal generation section 820 will bedescribed with reference to FIG. 28. As shown in FIG. 28, the softdecision baseband signal generation section 820 includes a plurality ofD flip-flops 822, 824, a plurality of multipliers 826, and an adder 828.

The received digitally modulated signal (hereinafter, received signal)and the sampling clocks Clk [0] to Clk [7] generated by the samplingclock generation section 800 are input into each D flip-flop 822. The Dflip-flops 822 sample the received signal with one bit accuracy at atiming when the input sampling clocks Clk [k] (k=0 to 7) rise, andgenerate sampling data. Further, the D flip-flops 822 hold the samplingdata until the next rising timing of the sampling clocks.

In other words, in the D flip-flops 822, the sampling data are updatedin synchronization with the cycle of the sampling clocks Clk [k]. Thesampling data obtained by the D flip-flops 822 are input into the nextstage D flip-flops 824. Further, the sampling clock Clk [0] (drivingclock) is input into the next stage D flip-flops 824. The D flip-flops824 are provided to synchronize the sampling data obtained by theprevious stage D flip-flops 822 with the driving clock.

First, the D flip-flops 824 obtain the sampling data held by theprevious stage D flip-flops 822 at a timing when the input samplingclock Clk [0] rises. Next, the D flip-flops 824 hold the obtainedsampling data until the next rising timing of the sampling clock Clk[0]. In other words, the D flip-flops 824 hold the sampling dataobtained from the previous stage D flip-flops 822 for one cycle of thesampling clock Clk [0].

By such processing of the D flip-flops 824, the sampling datasynchronized with the sampling clock Clk [k] (k#0) in the previous stageD flip-flops 822 synchronize with the sampling clock Clk [0].

To synchronize the sampling data with the sampling clock Clk [0],settling of the sampling data by the D flip-flops 822 has to becompleted at the timing when the D flip-flops 824 obtain the samplingdata. However, if the settling time is long because of a reason such aswiring delay, the settling of the sampling data may not be completed atthe timing when the D flip-flops 824 obtain the sampling data.

When such a situation is expected, it is preferred that an appropriatenumber of D flip-flops are provided between the D flip-flops 822 and theD flip-flops 824. By employing the above configuration and delaying thesynchronization timing of the sampling data, each sampling data can besynchronized with the sampling clock Clk [0] even when wiring delay orthe like occurs. The sampling data obtained by the D flip-flops 824 areinput into the multipliers 826. Predetermined coefficients K0 to K7 areheld in each multiplier 826.

First, the multipliers 826 obtain the sampling data from the Dflip-flops 824. The multipliers 826 multiply the obtained sampling databy the coefficients Ki (i=0 to 7) held in the multipliers 826. Outputvalues from the multipliers 826 (hereinafter, sampling data) are inputinto the adder 828.

The above coefficients Ki are set based on information of the samplingclock Clk [k] having a phase closest to the phase of the carrier wave ofthe received signal. For example, consider a case in which the phase ofthe sampling clock Clk [0] is closest to the phase of the carrier wave.When the received signal is a BPSK signal, the combination ofcoefficients Ki is set as, for example, (K0, K1, K2, K3, K4, K5, K6,K7)=(1, 2, 2, 1, −1, −2, −2, −1). In another example, the combination ofcoefficients Ki may be set as (K0, K1, K2, K3, K4, K5, K6, K7)=(1, 3, 3,1, −1, −3, −3, −1).

The adder 828 sums the sampling data input from a plurality ofmultipliers 826 and generates a soft decision baseband signal (RfData).The soft decision baseband signal here is a sum of values obtained bymultiplying eight sampling data sampled in one cycle of the carrier waveby the coefficients. When the combination of the coefficients Ki is setas (1, 2, 2, 1, −1, −2, −2, −1), the value of the soft decision basebandsignal will be between −6 to 6. The soft decision baseband signalgenerated in this way is input into the data generation section 830.

The sign of the soft decision baseband signal represents a bit value ofthe original data. For example, the positive sign of the soft decisionbaseband signal represents the bit value “1”, and the negative sign ofthe soft decision baseband signal represents the bit value “0”. Theabsolute value of the soft decision baseband signal represents theprobability of the bit values. Therefore, when the information of thesampling clock having a phase closest to the phase of the carrier waveis not obtained, by determining the coefficients K0 to K7 for everysampling clock, generating the soft decision baseband signals for everysampling clock, and comparing the absolute values of the soft decisionbaseband signals, it is possible to determine the sampling clock havinga phase closest to the phase of the carrier wave.

4-1-3: Circuit Configuration of Data Generation Section 830

Next, the data generation section 830 will be described with referenceto FIG. 29. In the data generation section 830, a data clock DClk anddata sampling clocks DClk [k] (k=0 to 7) described below are assumed tobe input.

The data clock DClk has the same cycle as that of the original data ofthe received signal, and is a periodical wave synchronized with thedriving clock. For example, the data cycle of the received signal is setto 8 times the cycle of the carrier wave. In this example, the cycle ofthe data clock DClk is set to 8 times the cycle of the driving clock.Here, The data cycle here is a cycle in which an interval assigned toeach data value in a code series into which the original data is encodedappears. The data cycle of the received signal is set to an integralmultiple of the cycle of the carrier wave. The data clock DClk isgenerated by dividing the driving clock by 8. On the other hand, thedata sampling clocks DClk [k] (k=0 to 7) are generated by delaying thephase of the data clock DClk. For example, the data sampling clocks DClk[k] (k=0 to 7) are generated by delaying the phase of the data clockDClk by k-eighth cycle.

As shown in FIG. 29, the data generation section 830 includes aplurality of D flip-flops 832, 834, a plurality of multipliers 836, andan adder 838.

The soft decision baseband signal generated by the soft decisionbaseband signal generation section 820 and the data sampling clocks DClk[0] to DClk [7] are input into each D flip-flop 832. The D flip-flops832 sample the soft decision baseband signal with 3 bits accuracy at atiming when the input data sampling clocks DClk [k] (k=0 to 7) rise, andgenerate sampling data. Further, the D flip-flops 832 holds the samplingdata until the next rising timing of the data sampling clocks.

In other words, in the D flip-flops 832, the sampling data are updatedin synchronization with the cycle of the data sampling clocks DClk [k].The sampling data obtained by the D flip-flops 832 are input into thenext stage D flip-flops 834. The data clock DClk is input into the nextstage D flip-flops 834. The D flip-flops 834 are provided to synchronizethe sampling data obtained by the previous stage D flip-flops 832 withthe data clock DClk.

First, the D flip-flops 834 obtain the sampling data held by theprevious stage D flip-flops 832 at a timing when the input data samplingclock DClk [0] rises. Next, the D flip-flops 834 hold the obtainedsampling data until the next rising timing of the data sampling clockDClk [0]. In other words, the D flip-flops 834 hold the sampling dataobtained from the previous stage D flip-flops 832 for one cycle of thedata sampling clock DClk [0].

By such processing of the D flip-flops 834, the sampling datasynchronized with the data sampling clock DClk [k] in the previous stageD flip-flops 832 synchronize with the data clock DClk. To synchronizethe sampling data with the data clock DClk, settling of the samplingdata by the D flip-flops 832 has to be completed at the timing when theD flip-flops 834 obtain the sampling data. However, if the settling timeis long because of a reason such as wiring delay, the settling of thesampling data may not be completed at the timing when the D flip-flops834 obtain the sampling data.

When such a situation is expected, it is preferred that an appropriatenumber of D flip-flops are provided between the D flip-flops 832 and theD flip-flops 834. By employing the above configuration and delaying thesynchronization timing of the sampling data, each sampling data can besynchronized with the data clock DClk even when wiring delay or the likeoccurs. The sampling data obtained by the D flip-flops 834 are inputinto the multipliers 836. Predetermined coefficients Q0 to Q7 are heldin each multiplier 836.

First, the multipliers 836 obtain the sampling data from the Dflip-flops 834. The multipliers 836 multiply the obtained sampling databy the coefficients Qi (i=0 to 7) held in the multipliers 836. Outputvalues from the multipliers 836 (hereinafter, sampling data) are inputinto the adder 838.

The above coefficients Qi are set based on information of the datasampling clock DClk [k] having a phase closest to the data clock DClk.For example, consider a case in which the phase of the data samplingclock DClk [0] is closest to the phase of the data clock DClk. In thiscase, the combination of coefficients Qi is set as, for example, (Q0,Q1, Q2, Q3, Q4, Q5, Q6, Q7)=(1, 1, 1, 1, −1, −1, −1, −1). In this case,a difference value between the sum of values obtained by sampling thesoft decision baseband signal in a half cycle located in the first halfof the data cycle and the sum of values obtained by sampling the softdecision baseband signal in a half cycle located in the second half ofthe data cycle becomes the soft decision data (output of the adder 838).

The adder 838 sums the sampling data input from a plurality ofmultipliers 836 and generates a soft decision data (EQData). Forexample, when setting the Qi as described above, the value of the softdecision data becomes between −48 to 48. The soft decision data obtainedin this way is input into the indicator calculation section 206 as thevalue detected by the soft decision data generation section 204.

The sign of the soft decision data represents a bit value of theoriginal data. For example, the positive sign of the soft decision datarepresents the bit value “1”, and the negative sign of the soft decisiondata represents the bit value “0”. The absolute value of the softdecision data represents the probability of the bit values. Therefore,when the information of the data sampling clock having a phase closestto the phase of the data clock DClk is not obtained, by determining thecoefficients Q0 to Q7 for every data sampling clock, generating the softdecision data for every data sampling clock, and comparing the absolutevalues of the soft decision data, it is possible to determine the datasampling clock having a phase closest to the phase of the data clockDClk.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alterations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

(Note)

The soft decision data generation sections 204, 404, 504, and 604 are anexample of a soft decision data detection section. The indicatorcalculation sections 206, 406, 506, and 606 are an example of anindicator calculation section, a standard deviation calculation section,and an absolute value average calculation section. The packet lengthmodification sections 210, 310, 514, and 614 are an example of a packetlength control section, a packet length calculation section, and apacketizing section. The signal transmission sections 412, 518, and 618,the packetizing section 408, and the packet length modification sections514 and 614 are an example of an indicator transmission section.

The present application contains subject matter related to thatdisclosed in Japanese Priority Patent Application JP 2009-039939 filedin the Japan Patent Office on Feb. 23, 2009, the entire content of whichis hereby incorporated by reference.

1. A communication apparatus, comprising: a soft decision data detectionsection which detects soft decision data from a received digitallymodulated signal; an indicator calculation section which calculates anindicator indicating a degree of variation in the soft decision datadetected by the soft decision data detection section; and a packetlength control section which controls a packet length in accordance withthe indicator calculated by the indicator calculation section.
 2. Thecommunication apparatus according to claim 1, wherein the indicatorcalculation section includes: a standard deviation calculation sectionwhich calculates a standard deviation of the soft decision data of someor all of packets included in the digitally modulated signal; and anabsolute value average calculation section which calculates an averagevalue of absolute values of the soft decision data of some or all of thepackets, and the indicator calculation section calculates the indicatorbased on the standard deviation calculated by the standard deviationcalculation section and the average value calculated by the absolutevalue average calculation section.
 3. The communication apparatusaccording to claim 2, wherein the indicator calculation sectioncalculates the indicator S=σ/(2*E) based on the standard deviation σ andthe average value E.
 4. The communication apparatus according to claim3, wherein the packet length control section shortens the packet lengthwhen the indicator S is large, and lengthens the packet length when theindicator S is small.
 5. The communication apparatus according to claim4, wherein the packet length control section refers to a predeterminedtable configured to associate predetermined packet lengths with theindicators S, selects a predetermined packet length in accordance withthe indicator S calculated by the indicator calculation section, andpacketizes data so that the data has the predetermined packet length. 6.The communication apparatus according to claim 4, wherein the packetlength control section includes: a packet length calculation sectionwhich uses a predetermined calculation formula for calculating anexpected value of a time used to transmit all data of n bits which istransmitted by dividing the data into packets of k bits (n>k) when acommunication condition corresponds to the indicator S, and calculates apacket length k which makes the expected value minimum; and apacketizing section which divides transmission data based on the packetlength k calculated by the packet length calculation section inaccordance with the indicator S calculated by the indicator calculationsection and the number of bits n of the transmission data, andpacketizes the divided transmission data.
 7. The communication apparatusaccording to claim 1, further comprising an indicator transmissionsection which transmits information of the indicator calculated by theindicator calculation section to a transmission source of the digitallymodulated signal.
 8. The communication apparatus according to claim 4,further comprising a communication control section which stops datatransmission processing when the indicator calculated by the indicatorcalculation section is greater than a predetermined threshold value, andre-starts the data transmission processing when the indicator is smallerthan a predetermined threshold value.
 9. A communication apparatus,comprising: an indicator reception section which receives an indicatorwhen soft decision data of a digitally modulated signal is detected in atransmission destination of the digitally modulated signal transmittedby the communication apparatus, and the indicator representing a degreeof variation in the soft decision data is calculated and transmitted tothe communication apparatus; and a packet length control section whichcontrols a packet length in accordance with the indicator received bythe indicator reception section.
 10. The communication apparatusaccording to claim 9, wherein the packet length control section shortensthe packet length when the indicator S is large, and lengthens thepacket length when the indicator S is small.
 11. The communicationapparatus according to claim 10, wherein the packet length controlsection refers to a predetermined table configured to associatepredetermined packet lengths with the indicators, selects apredetermined packet length in accordance with the indicator calculatedby the indicator calculation section, and packetizes data by thepredetermined packet length.
 12. The communication apparatus accordingto claim 11, wherein the packet length control section includes: apacket length calculation section which uses a predetermined calculationformula for estimating an expected value of a time used to transmit alldata of n bits which is transmitted by dividing the data into packets ofk bits (n>k) when a communication condition corresponds to the indicatorS, and calculates a packet length k which makes the expected valueminimum; and a packetizing section which divides transmission data basedon the packet length k calculated by the packet length calculationsection in accordance with the indicator S calculated by the indicatorcalculation section and the number of bits n of the transmission data,and packetizes the divided transmission data.
 13. The communicationapparatus according to claim 10, further comprising a communicationcontrol section which transmits a digitally modulated signal including apredetermined packet when the indicator received by the indicatorreception section is greater than a predetermined threshold value.
 14. Acommunication system, comprising: a first communication device includinga signal transmission section which transmits a digitally modulatedsignal to a second communication device, an indicator reception sectionwhich receives an indicator transmitted from the second communicationdevice, and a packet length control section which controls a packetlength of a packet to be transmitted to the second communication devicein accordance with the indicator received by the indicator receptionsection; and the second communication device including a signalreception section which receives the digitally modulated signaltransmitted from the first communication device, a soft decision datadetection section which detects soft decision data from the digitallymodulated signal received by the signal reception section, an indicatorcalculation section which calculates an indicator indicating a degree ofvariation in the soft decision data detected by the soft decision datadetection section, and an indicator transmission section which transmitsthe indicator calculated by the indicator calculation section to thefirst communication device.
 15. A packet length control method,comprising the steps of: detecting soft decision data from a receiveddigitally modulated signal; calculating an indicator indicating a degreeof variation in the soft decision data detected in the step of detectingsoft decision data; and controlling a packet length in accordance withthe indicator calculated in the step of calculating an indicator.
 16. Apacket length control method, comprising the steps of: receiving anindicator when soft decision data of a transmitted digitally modulatedsignal is detected in a transmission destination of the digitallymodulated signal, and the indicator representing a degree of variationin the soft decision data is calculated and transmitted to atransmission source of the digitally modulated signal; and controlling apacket length in accordance with the indicator received in the step ofreceiving an indicator.
 17. A packet length control method, comprisingthe steps of: by a first communication device, transmitting a digitallymodulated signal to a second communication device; by the secondcommunication device, receiving the digitally modulated signaltransmitted from the first communication device; detecting soft decisiondata from the digitally modulated signal received in the step ofreceiving the digitally modulated signal; calculating an indicatorindicating a degree of variation in the soft decision data detected inthe step of detecting soft decision data; transmitting the indicatorcalculated in the step of calculating an indicator to the firstcommunication device; by the first communication device, receiving theindicator transmitted from the second communication device; andcontrolling a packet length of a packet to be transmitted to the secondcommunication device in accordance with the indicator received in thestep of receiving the indicator.